Why lab sessions are handled by TAs?

TAs exist because they are cheaper than professors.

Superbee wrote

The main reason that TAs or lab instructors are used in these subjects is because hands-on instruction does not scale as well as lecture-style instruction.

This is a euphemistic way of saying TAs are cheaper.

Sometimes it is claimed that TAs exist so that the TAs can learn to teach. This is not true. They exist because they are cheaper. Sometimes they do learn something about teaching, and this may be part of their compensation.

As suggested by Dmitry Savostyanov designing and teaching a course requires expertise. Therefore, TAs do not take lectures since they are not very experienced in teaching. This leaves us with the remaining work that must be distributed to decrease the workload on an instructor. Therefore, the TAs are assigned the coding and grading part. Generally, these assignments are focused on a specific concept so a TA do not need to know everything about the course.

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Note: Sometimes professors also ask TAs to take a tutorial or a small lecture so that they can acquire some teaching experience and skills. I am lucky that professors in my department do so.

I think your question extends beyond programming-related courses and also applies to virtually any course with a significant hands-on laboratory component (think chemistry labs, biology labs, physics labs, etc).

The main reason that TAs or lab instructors are used in these subjects is because hands-on instruction does not scale as well as lecture-style instruction. A professor can easily lecture about an algorithm to hundreds of students at the same time. But can the professor also help hundreds of students implement and debug the algorithm on their computers? Not very efficiently.

This is why TAs and lab instructors are hired. They can provide more individualized guidance and instruction with hands-on learning than a single professor probably ever could, and they are much cheaper.

First, I would challenge your assumptions that professors are not involved in the coding part. Even if they are not directly involved in delivering the labs, many professors are in fact involved in designing the lab classes, including problems that are offered to students. Sometimes professors also include suggestions on how to deal with most common learning obstacles, how to respond to most common questions, and which hints to give at which situations. Professors also design assessment problems and the marking scheme, i.e. instruct TAs onhow to evaluate the codes submitted by students. To summarise, there is a lot of work behind the scenes, which is Professor's responsibility.

Designing the course requires a lot of high-level expertise and understanding of the subject. It takes certain mastery to structure the material and arrange it in a way which keeps students involved, interested, and appropriately challenged along the course. To design a course which runs smoothly, professors have to create a sequence of effective teaching events which spans several weeks / months and involves tens / hundreds of students. TAs usually are given precise instructions for each class and plan how to effectively discuss the given problem in a small(er) tutorial group, or how to answer questions of a single student. Although this work also requires some creativity, it's more scoped and therefore easier to plan and prepare to.