For better integration with an issue tracker, and because a master branch should have a formal commit history.

I make tons of little commits on a local branch with messages such as "reformat styles", "whoops, fixed typos", "fixed the query", and such. I use "rebase -i" to squash all of them together with a better, and more formal, commit message, like "Repairs and styles main navigation, closes #9".

I hate a master branch with a commit history with casual commit messages.