Majority of the boots that are out on the market just prevent your horses legs from knocks, knicks, bangs and scrapes.
If you watch alot of Gymkanner's, the majority use boots on their horses legs when involved in these highly physical games. What I would use if I were doing sports such as those involved in Gymkanna, is a boot that wraps around the entire leg, protecting the full amount from front, side to back - to ensure that no matter where the horse gets hit either from objects that you are galloping around, or from him/herself.
You can also use these when you merge into Jumping.
You don't have to go out and buy expensive boots, heck, I just bought myself a set of 4 boots just like this:
In black, for $40.00 from my local tack store. They do their job, lined in neoprene and I like that I got black so that I can go with whatever colors I choose for the day.
Uses - to protect your horses legs from getting knocked, knicked, bumped, scraped. Some aid in absorbing shock, just like Athletes Running Shoes that are designed in this manner, so are some athletic horse boots.
When - I only use mine when I am Jumping or out on the CC course, or out on a long hack/Fox Hunt/Hunters Pace.
I do not use boots on my boy when I am doing dressage/flat work, out on a conditioning hack.
If your horse does not interfear with him/herself while doing patterns at home, then I wouldn't worry about wearing boots. *Interfear meaning legs knocking themselves, hind legs hitting the front legs, etc, etc* But if your horse does have a tendancy of interfearing with him/herself, then I would consider putting boots on to help protect the lower legs.