It adds horses that you can ride! Truly a wonderful reward for finding and taming a horse. It makes exploration and transportation so much better. It really opens up the map, especially if you don't want to go the teleportation route.
Make sure you use horse shoes though. Horse shoes make the horses go faster! If you don't know that, you'll be stuck with a slow horse.
Also, I found that the horses were still a bit slow. If you are the same way, I recommend upping the "speed" rating of the horseshoes by 1.5 times, so (steel,mese,diamond) = (7,9,10) becomes (10.5,13.5,15)