Because the professor doesn't scale, not in an introductory class with > 100 students. Most all problems with the material that arise there can be dealt with with the help of the TA. At graduate level it's different, of course.

this presumes that:

- professors have open hours

- professors can be found before or after the lecture

- professors respond to your email

- professors ever go to their offices

A competent TA might have more of a rapport with a student, since they'd be closer in age and experience. It's likely that a grad student TA has recently taken the same classes and done the same kind of assignments. Also, a TA may have more time to spend with a struggling student than a professor has. (I've been a TA.)

One thing that hasn't been mentioned - sometimes it helps to have a second voice explaining things, if you didn't get it the first time. If there's something about the professors explanation that is the problem, even if they're accessible it may not help.

A good TA can help approach things from a different direction.