They provide protection of the legs, tendons, ankles etc. in case the horse should kick himself or get in a fight with a jump.
Polo wraps are fine for eventing as long as you tape them so the Velcro stays secure. The issue would be a water obstacle when they could get wet and loose which is why most eventers use boots. I'm a SMB fan, they seem to fit most horses and they hold up pretty well. What you're looking for in leg protection is something that fits well and is durable enough to handle jumping specifically cross country when the jumps don't move.
I prefer polos for dressage, I think they look kinda classy :)