Well, almost all horse that are shod have necrotic (dead) tissue that has built inside the hoof. Horses that are shod have decreased blood flow (circulation) because the shoes prevent the horses hoof from naturally expanding when it hits the ground, and this action "pumps" blood throughout the hoof. And, so when the horse is shod, the circulation can't flush out this dead tissue in the hoof, and when you pull your horses shoes, the abscess is the horses way of getting rid of the necrotic tissue.
Do not take that as a message to shoe your horse year-round. I would seriously consider going barefoot. If you have a horse that tends to be ouchy on rocks, use rubber hoof boots. They usually cost around $100, and last 5 years or so. After 1-2 trims, your hoof boots will already start paying you back! Horses that are barefoot live longer because of increased circulation in the hoof. Since the heart isn't trying to pump blood into the hooves but not being able to, the horses heart will be healthier. There is less strain on the joints because, again, that natural flexing of the hoof that only barefoot horses can do provides a cushion to the horse when they are walking. And not only that, the horse's frog can also do its job!
“Good things come to those who wait… greater things come to those who get off their ass and do anything to make it happen.” - Unknown