Fragging corals is a big part of the hobby. Being able to acquire frags from others has benefits. Corals are shared rather than the constant harvesting of wild colonies in the ocean, and they are already accustomed to dwelling in a closed ecosystem. The giver prunes the colonies in their reef, allowing for better flow. They are able to 'bank' a coral with you in case theirs perishes for whatever reason. The receiver enjoys watching the little piece grow into something beautiful over time, and then will one day be a giver as well.
Over the years, commercial sales have shifted from colony-sized corals to frag-size. Getting bigger colonies is nearly unheard of. Just about anything can be fragged, which is to cut a fragment off the bigger colony. I have a general rule: I won't frag a frag. I'm more of a hoarder, preferring to grow them into big colonies but from time to time I have to cave in and snip a few piece here and there.