It doesn't. Joel is a little bit off base here, in that it makes no difference whether a VCS tracks snapshots or deltas. What does matter is whether it keeps track of merges. If you merge one branch into another, your VCS must record that information, so that future merges can be done correctly. SVN doesn't do this, Git and Mercurial do. (And note that Git stores snapshots, not changes.)

Joel's absolute right about one thing, though. Being able to merge correctly and reliably make a huge difference. It's what makes the distributed part of DVCS possible. Without a central repository, branches happen a lot more often, and merging has to work right. The contortions that teams go through to prevent branches are a thing of the past.