<p>assuming you’re talking about an undergrad – access to the professor (and hence good letters of rec), the recognition and responsibility may look good to grad schools or future employers.</p>
<p>for a grad student – its typically a requirement in order to get paid. Grad school (PhD programs) is typically free at better schools, often even pays you to attend! The quid-pro-quo is a requirement to spend a certain amount of time as a TA or RA.</p>