Why do base kilometers help with time trials?

Question

Question:
If I want to maintain some objective measure of improvement by keeping track of the time it takes me to do a 40-km round-trip, why is it important to not do a time trial every time (riding every other day), but to vary the workout by introducing longer and shorter trips, and, particularly, to often do the 40-km round-trip at a leisurely pace?
Context and Unlikely Answers:
I have no intention of racing. I'm using time trials as a loose measure of progress mainly because other objective measures (maximum heart rate over the trip when completed in less than a given time; the much more complicated oxygen efficiency measurements) are not quite so straight-forward.
I don't see the parallel in other sports. For instance, 100m/200m sprinters would not practice by running marathons, nor even 1/10th of marathons.
A tennis player who wants to become a better player does need to play leisurely often (with less challenging partners, for example), but that's because the reflexes then improve, making tougher games more easily winnable (by instinct). Improving reflexes is important in cycling, for things such as avoiding falls, but it's unclear why they might help with time trials.
It would be nice to understand what exactly happens when one rides at a leisurely pace, that eventually improves time trials. Do, for example, glycogen stores in muscle cells become gradually capable of storing more (glycogen), hence enabling a more intense workout during a time trial?
Sequels:
Like all good answers, what Michael wrote generates many more questions to help us increase our understanding of the "why" part:

If you've gone to any aerobics class, you'll have noticed that it's a very bad idea to maintain your heartbeat at 170 and immediately leave the studio with a heartbeat dropping down to 70. The workout will take a heavy toll on your body. Instead, it's important to slow down gradually to return your body to the lower energy state. Is something similar happening with cycling? If true: Suppose, for example, that I aim instead for an intense 10-km round-trip (every other day), but do not return until I have cycled 30 more kilometers at a leisurely pace. Does this also count as "periodization"?

Is the idea to simply keep moving at lesser power? Suppose I have a rowing machine sitting unused, or suppose I dabble (also willingly) with tennis. Can I use these alternative activities as the "low power" activities—during rainy days for example? In other words, do the base kilometers really have to be cycling kilometers, rather than running on the court or (virtual) rowing kilometers?

What if I'm not at all aiming for a (yearly) race day? These Tour de France cyclists look like they're a different breed of humans anyway. I just want to maintain a regular, gradual, and possibly very slow, level of improvement. Can I set up my own monthly race day? Is that inherently sub-optimal than a yearly race day? (Or, really digressing, is one yearly race all that the human body can aim for? In particular, do TdF cyclists use the Giro and the Vuelta as merely practice rides? Do they save their real energy for the Tour?)

If you live in Southern Italy and are enjoying cycling year round, you may not quite appreciate the frustration of those of us who have a far shorter season during which to enjoy the thrill. You may not even fathom why we sometime have to think about other than cycling.

Argenti Apparatus · Answer

I think what you are asking here is what's the better training routine:

Ride 40 km at max effort (for that distance/time) - i.e. a 'time
trail' every time
Ride some shorter more intense rides mixed in with lower effort
'leisurely ' rides.

The basic idea is that you can make better improvements by training at higher intensity for shorter periods, but you then need recovery periods for your body to adapt. Many riders choose to do low intensity rides during these periods.

Michael · Answer

It’s all about maximizing the positive effects of training while minimizing recovery time and injury risk
You want to target different systems in the body as effectively as possible. A naive approach to training would be to (try to) train at race intensity and duration each and every day. However you’ll quickly reach a point where you can’t recover fast enough, suffer from injuries or reach a point where you don’t improve anymore. Different systems in the body also respond to different intensities.
Long rides are great to strengthen the heart muscle, improve oxygen uptake of the muscles and improve how fast you can burn fat. Training these systems mostly depends on training duration and less on intensity. Long and easy can even be done after hard training days and shorter easy rides can improve recovery because of increased blood flow through the muscles. Short, high intensity training is best to improve other factors like VO₂Max or pure muscle strength.
Apparently until a few decades ago athletes used to perform the same workouts, day in day out, the whole year long. Periodization was kind of a break-through. Now you generally start the season with a lot of long, easy endurance workouts and replace them with more and more relatively short, high quality, high intensity workouts the closer you get to the final race day.
Edit: I’ll try to answer the follow up questions:

I remember an episode on That Triathlon Show where an expert cited research that cool-down (i.e. continuing at very low intensity after a hard workout) actually doesn’t have any positive effect for recovery or training effectiveness. She mentioned that the only positive effect is that it prevents people from fainting after a race because it prevents the blood from suddenly pooling in the legs (I imagine simply lying down would achieve this as well). Not sure why people still seem to do it, maybe because it’s also a good way to calm down.
At least for running, Daniel Craig’s ”Running Formula” recommends to do running if you want to get better at running. I think he mentions that off-season activities like cross country skiing can have a positive effect but he doesn’t recommend it if all you want to do is get better at running. I think the same applies to bicycling though since it’s less dependent on form/technique it probably benefits more than running from exercise which targets similar muscles, endurance etc.
I don’t know about that one. I think your question boils down to if doing a certain type of workout for a few days or weeks in a row (e.g. several weeks of easy rides during base training phase) is more effective than simply throwing it in every now and then. Some training plans do have supercompensation blocks where you do high intensity workouts for a few days in a row (more or less intentionally going above the maximum recoverable volume). Not sure you could fit that in a monthly training cycle.

Andy P · Answer

It would be nice to understand what exactly happens when one rides at a leisurely pace, that eventually improves time trials. Do, for example, glycogen stores in muscle cells become gradually capable of storing more (glycogen), hence enabling a more intense workout during a time trial?

In response to this part of the question, there is a well known table of adaptations for each training zone.

As can be seen, small improvements can be made in many areas even at low intensities.  However small improvements can add up to big improvements when you factor in that you can ride (and recover from) a much higher volume of low intensity training

Why do base kilometers help with time trials?

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