Seems like a lot of extra effort (save, add, commit, come up with some message even if it's a prayer to work now) only to undo it again later and create a patch or alternate history out of the final version. Why bother with the intermediate commits if you're not planning for it to be part of the history?

Git is a version control system. It does not care about what it versions.

When I work on something, I commit often and use the commit graph as a undo tool on steroids. I can see what I tried, I can cherry-pick or revert stuff while experimenting, I can leave promising but unfinished stuff to look at later, or I can just commit to have a simple way to send stuff to CI, or a remote backup synced between machines.

Once I'm done working on something, it's time to take a step back, look at the big picture, see how many changes my experiments have actually yielded, separate them, describe and decide whether they go to review together or split in some way, as sometimes working on a single thing requires multiple distinct changes (one PR with multiple commits), but sometimes working in a single session yields fixes for multiple unrelated issues (several PRs). Only then it gets presented to the reviewer.

It just happens that I can do both these distinct jobs with a single tool.

Because I might want to go back to this current messy state but I don't want to commit it like this (hardcoded test strings, debug logs, cutted corners to see if something works, you name it).

I simply commit something like "WIP: testing xy" and if its working and properly implemented i can squash/rebase/edit the commit message and force push it to my feature branch. Using a Git client like Gitkraken makes this incredibly easy, takes seconds.

This way I can leverage version control without committing bogus states to the final PR.

If the team is using a PR workflow, the PR is a working place to produce one single commit. The individual commits are just timestamped changes and comments. Think of it as the equivalent of annotated diff in mailing list conversation.