It's a pretty esoteric feature, I'll grant you, but it's needed to implement several useful, more user-oriented features. The big one is deferred fields, which in certain situations can give apps enormous performance gains. If you're interested in how these features are related, Malcolm lays it all out here: http://www.pointy-stick.com/blog/2009/03/23/yak-shaving-adva...

It's important to understand that often the simplest features have the most complex underpinnings. In other words, simple APIs often require complicated internals.