Since it's unlikely to affect many people, you don't want to force everyone to download a whole new release, but you'd like to be able to make just the differences available as a patch for those who need it. We need to be able to create that patch easily. Revision control makes it easy to create patches. But it's not possible to use directly, since their patch is for an earlier release of the program, and you've made quite a few changes. We'd like to be able to integrate changes from different versions more easily than just re-coding the entire change.
Revision control makes integrating changes easier.
A developer who lives on the other side of the world is sending you patches almost every day. It's great to get all this help coding, but you're having trouble integrating all their changes, since you're not quite modifying the same code. If only you could share the same source more easily. Revision control can make working in a group easier. A change that was intended to speed up the program instead just slowed it down.