Drawing charges is definitely more valuable than a block IMO, but a steal is debatable. Most blockers in today's NBA (Dwight Howard, etc) just block the ball out of bounds, which isn't particularly useful. A steal on the other hand often times leads to a fast break.
Davis' ability to draw charges is far and away his best defensive attribute. I think the coaches probably felt that he was often times a liability on defense, so instead of just improving overall defensively, they also asked him to get this niche ability to very good/useful levels. He's also big enough to repeatedly take the hits, quick enough on his feet that he can get there in time, and hefty enough to absorb the fall. He really has gotten this down to near perfection, and its fun to watch.