Now, as a law professor, obviously this advice seems to be a bit self-serving. And maybe it is (though I already have an RA, so it doesn't do me any good for more people to want to be RAs). But I think it's good for students too, for at least two non-obvious reasons:
(1) It gives you an advisor. I attended an undergraduate institution where every student was assigned a faculty advisor. When I got to law school, I just kind of assumed the same policy would be in place. Spoiler: it isn't. Nobody is your advisor, and there isn't any obvious opportunity to find someone to answer the generic advising questions that you'll have over the course of your law school career: "Should I do law review?" "If I work at a firm, am I stuck there forever?" "Is it bad if I take a class that sounds interesting but doesn't have any 'real world' applications?" Being an RA gives you ready-access to a professor and is easily converted into an advising relationship. In general, if you ask if you can run through some general questions in the ten minutes following your RA meeting, they'll be more than happy to oblige.
(2) It is of massive help in writing letters of recommendation. I will confess: it is very hard to write letters of recommendation for students whom I only "know" because of their presence in my doctrinal classes. Even if they perform well, I have basically two data points about them: (a) their exam performance, and (b) the one time they were cold called (which probably happened in the second week of term and has long since faded from my memory). It's really difficult to take that and write a letter that doesn't seem horribly rote and formulaic. By contrast, professors get to know their RAs personally and can speak about their capabilities in a much more specific and intimate fashion. It is very likely that the professor whom you RA for will become your strongest letter writer, even if they didn't give you your highest transcript grade.
All of this exists on top of the more obvious reasons to become an RA (enjoying research, being interested in the subject matter, resume line, etc.). But becoming an RA is the easiest and fastest way to develop a close relationship with one your professors, and that benefit is of incalculable worth as you move through law school.