What is the benefit of git's two-stage commit process (staging)?

Question

I'm learning git and I've noticed that it has a two-step commit process:

1. git add <files>
2. git commit

The first step places revisions into what's called a "staging area" or "index".

What I'm interested in is why this design decision is made, and what its benefits are?

Also, as a git user do you do this or just use git commit -a?

I ask this as I come from bzr (Bazaar) which does not have this feature.