Apply changes from one branch to another.

Typically when you have used branch to enable your development to diverge, you later want it to converge again, at least partially. For example, if you created a branch for 3.0.x bug-fixes, you probably want those bug-fixes to happen in the main line of development as well. Without the merge operation, you could still achieve this by manually doing the bug-fixes in both branches. Merge makes this operation simpler by automating things as much as possible.