My opinions on whether to squash merge or use merge commits haven’t been consistent over the past few years. Early on, I was mostly in favor of squashing commits, then I decided that in order to use certain tools like git bisect, the complete commit history for a given branch should remain intact.
But, my experience over the past year or show has shown me that preserving this history isn’t very useful, or at least hasn’t been very useful for me.
While it can be useful to see how a branch has progressed over time; it has been easier for me to rely on the discussion in the pull request to determine why certain decisions might have been made. By making a strong effort to keep pull requests small and narrowly focused, it doesn’t seem as necessary to preserve the development of the pull request inside of version control - that is, the pull request hopefully provides enough context for me to understand what that pull request is doing without me needing to investigate the commits of which the pull request is comprised.
The main advantage that I see from using a squash strategy, is that the commit history for a project can be traversed and understood more quickly - each commit on the master branch should have have tests that pass and code that functions correctly (correctly in the sense of as designed at the time).
In my opinion, squashing just presents a cleaner and more navigable history to developers than using merge commits. I find that more beneficial than being able to understand why a developer decided to commit work at a given point.