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Readers, who grade course assignments, are not the same as TAs. Much fewer responsibilities for readers. TAs need to lead discussions and hold office hours. This is on top of their research commitment and their own coursework. Some departments allow exceptional 4th year undergrads to be TAs. I personally don’t like this idea though</p>
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A TA-ship is required for PhD candidates in my department. Other than this requirement, there is a lot of funding associated with a TA position. It was either free tuition or free housing, I don’t remember, though it is likely to be the latter. Usually professors would like to save their fellowship money for other students, so in order to fund their PhD students, they can make them TAs once they are qualified. Within engineering, there are very few M.S students who can become TAs, most are PhD candidates. And absolutely no undergraduates.</p>