When I was first learning to code, it took some effort for me to understand what a function did, particularly the concept of returning an expression, or what happens if you write `foo() + bar()`. (In what order are `foo` and `bar` called?)
Now of course those concepts are trivial. But it’s easy to forget their initial difficulty when teaching, so I might be liable to just breeze through functions and leave most students behind. A freshly-learned TA would remember what was hard and be able to tell students the intuition-pumps that helped them, while directing more advanced questions to the professor.
For example, a TA might know to explicitly say that every time you call a function, it remembers where it was called so it can `return` there at the end of the function’s execution. Then if a student wants more detail about that, they can ask the professor who can explain how a function call is invisibly being converted to code that allocates a new stack frame and sets the execution pointer.