I do it for several reasons: 1) I love horses and I want to spend as much time with them as I can, even if they're not mine and I'm not getting paid, 2) I love to promote horse sports to others and share my love of horses with others who feel the same way, and 3) I also enjoy helping out and promoting horse sports to those who may not be as involved or as knowledgeable as I am. Reason #3 is especially important to me because I want people who are new to the industry to feel welcome. The more, the merrier, after all!
In my case, the main reason is to know more about the equestrian sport. I'm always 100% in every activity my stables organizes like events, clinics or whatever... because you know how it works from the inside and it helps you to understand how much effort and time it takes. :)
I think volunteering is a good way to be involved in amazing events, you're not just a spectator but you're really useful. You can see the event from inside, learn a lot of things, meet people with the same passion,...