The GRE is kinda the "go to" test for most grad schools that aren't professional school. That is, law school requires the LSAT; biz school requires the GMAT; med school requires the MCAT. The GRE is there for pretty much everything else.
Want to waste your time and get a PhD in English so you can one day pour coffee for writers at Shmoop? Great. Take the GRE. It's a lot like the SAT, only with harder questions and stranger transmogrifications...and if you know what that word is, then maybe you too can pour.
We like two sugars, thanks.