Something about the sight of a scab brings on the urge to pick at it. You may even think you'd be doing a good thing by pulling the crust off a wound and "letting the air get to it." But there's no benefit to removing your horse's scabs, and it can even be harmful.
A scab is nature's bandage, acting as a protective layer against contamination. Peeling off scabs not only increases the chance of infection, but it can lead to scarring. What's more, picking at "tight" scabs---such as those resulting from rainrot, scratches and other inflammatory skin conditions---is extremely painful for a horse, which means you run the risk of getting kicked or nipped.
Leave scabs alone and groom the area around them gently so you don't accidently pull one off. If removing scabs is necessary to treat a skin condition, soften them first by applying a thick layer of ichthammol, Desitin or baby oil daily for a couple of days, gently wiping away the excess with a towel before each new application. The scabs will eventually soften and come off easily, allowing you to treat the underlying infection.