Why do building codes for basements apparently require "windows large enough for escape"?

It is to provide a secondary escape path in case of fire or other disaster. Same as houses must have two or more doors to the outside.

Fire code typically requires every room to have 2 exits. Basements without windows have only one exit: the stairs back to the main level. If a fire occurred at the stairs, persons in the basement would be trapped and unable to exit.

"Escape" simply means to avoid something dangerous, not necessarily evade captivity as you suggest. In a basement with no egress windows and only one set of stairs, if the stairs become blocked or unusable (due to a fire or collapse or anything else), one can become trapped in the basement, no malicious kidnapper required. In modern building codes, you need a secondary means of escaping a basement during a fire or other emergency, which are windows large enough to fit through.

I'm no building inspector, but I'm quite certain that a barred window would not count as an escape window no matter how large it is. The whole point is to have a secondary exit, but a blocked window doesn't fulfill that requirement.

Just to clarify a bit. The rule (generally, there may be exceptions in some places) is not that "every basement have two means of exit". If that were the case then, carried to the logical extreme, there could never be a room (beyond perhaps a small closet) anywhere that only had one way to get out.

The rules specifically apply to sleeping rooms (a.k.a. bedrooms). When everyone is awake, it is, hopefully, not too hard to escape in most cases through the primary, and often only, exit. The problem is when people are asleep a fire (or other problem, but fire is the biggest concern) could start and block the primary/usual means of exit, trapping people in bedrooms. Because of that, any bedroom must have a second means of exit. That could be a door direct to the outside or it can be a window that is large enough (there are minimums for width, height and total area) and that is not blocked (bars, obviously, but also a too-small window well or other things could cause problems).

Above-grade bedrooms are generally not a problem because most people want to have large windows anyway for light and air. High-rise buildings are, of course, a problem since windows are often not openable and, even if they are, dropping down more than about 2 or 3 stories is not generally very practical. That is one of the reasons (the other is risk of fire spreading to other apartments) why high-rise residential buildings more typically (varies by area) have sprinkler requirements than single-family homes.

Note that this is an extremely common problem in the real estate business. People will build out a nice bedroom in the basement with minimal (smaller than code requirement) or no windows. Those bedrooms can be a major safety hazard and can't be counted as bedrooms. So you might find a house listed as a 3-bedroom house but when you go inside you find that it is, for all practical purposes, a 4-bedroom house. That 4th bedroom in the basement likely doesn't meet the code requirement for an egress window and is, therefore, use at your own risk. Storage, home office, workroom - no problem. Bedroom - potential serious danger.

Serious house fires don't happen as often as they used to, but they do happen. Many years ago (15 - 20, can't remember exactly) there was a series of fires in my neighborhood. Fortunately only property damage - in one case so much that the house had to be torn down. One fire was due to electrical wiring. Another was candles. Another was lights catching bedding on fire. All different causes. All serious. All situations where getting out quickly mattered a lot.

One thing that can happen when reading regulations is "confirmation bias". Or to be more precise, the tendency to interpret the regulations in a way that doesn't conflict with your plans.

So it's natural to miss the fact that the "escape" requirement applies to occupants escaping fire.

The problem is, with bedrooms, you are fairly likely to be asleep while the fire takes hold, so you awake to face a fire that is fully engaged. With work spaces you will surely smell the smoke early enough to escape.