One of my riders signed up for it and carefully logged her hours.She sent in her reports but we have not heard back from USEF yet. I think the main benefit is getting your riding success acknowledged by your school and classmates.
I did it, and I have a letter or pin or something that the USEF sent me. For me, the benefit was that I could more easily define my riding on the common app by saying, right next to varsity swim team, that I was did "varsity equestrian" and explain my riding commitments in a way that was neater and easier for college admission officers to understand (wether or not it was actually a benefit....you'd have to ask the college admissions officers at a variety of small universities and liberal arts colleges from New England to Texas).
“Don’t ask yourself what the world needs; ask yourself what makes you come alive. And then go and do that. Because what the world needs is people who have come alive.” -Howard Thurman
I am planning on signing up this year-should get started on that! One of my friends is currently enrolled, and like the other posters said, it makes it easier for colleges and the high school to recognize it as a 'sport'.