A normal push will only advance the remote branch if your local branch has the tip of the remote branch as an ancestor. This ensures you're only adding commits

A normal push will only advance the remote branch if your local branch has the tip of the remote branch as an ancestor. This ensures you're only adding commits to the remote branch and not replacing any commits on the remote. But if you've pulled the remote branch, rebased, and now wish to push, the tip of the remote branch is longer an ancestor of your local branch. In this scenario, to update the remote branch you have to do a force push. But now imaging that another developer has added new commits to the remote branch in the mean time. If you do a force push, you will overwrite those new commits. Using "--force-with-lease" ensures that the tip of the remote branch hasn't changed since you last pulled it, so that your force push isn't erasing any commits on the remote by accident (i.e. it ensures the remote branch has not changed in between when you last pulled it and your current force push.)