Somebody claiming to be Alexander Kjeldaas wrote:

Here you might want to do a
git rebase -i <when-you-split-from-main-branch>
and squash unnecessary commits into larger ones.

I've never understood why people do this. It seems to violate the purpose of making good atomic commits in the first place.

Ideally people should always make nice commits as they work on code. In practice that rarely happens. For example, sometimes when you work on some change you might see some unrelated problem and fix it then and there, so you don't forget. Rebasing allows you to fix that up later.