For me, it's because I hop between development machines, and pushing/pulling a branch is much easier than the alternative of synchronizing files manually among said machines.

Also, so that if something goes awry with my dev machine for whatever reason, at least my work is saved.

Also, to make it easier for a colleague to review my code before it gets merged into something.

Also, becuase it means I can use GitHub's PR system instead of doing it on my machine (thus providing some additional record that my code got merged in, and providing an avenue for the merge itself to be reviewed and commented on).