Because someone else might need to work on something on or from my branches. And I don't want garbage in my history. There are cases I might not be able to squash merge, so all my history will be in the project history. I want each commit to be clean. It is not a lot of effort, and forces me to increase code quality, because I review my code more often.

Rules for all projects I manage: never rebase published branches and always publish clean code (even implementation is unfinished).

From experience following these simple rules make the whole project management easier and more effective