Branches as Savepoints

Because a Git branch is just a 40-byte file on disk, it takes orders of magnitude more time for you to tell the computer to create a branch (by typing git branch foo) than for your computer to actually do it.

And because branches are references, and (say it with me now) references make commits reachable, creating a branch is a way to "nail down" part of the graph that you might want to come back to later.

And because neither git merge nor git rebase will change your existing commits (remember, a commit's ID is a hash of its contents and its history), you can create a temporary branch any time you want to try something you're even just a little bit unsure about.

In other words, creating a branch is like saving your game before you battle the boss.