Is it always possible to enforce PEP8 with a bot, and no human intervention?
Is it always useful to strictly enforce PEP8?

I already saw cases when e.g. the rule for length of lines was broken on purpose and this helped the readability.

PEP8 is basically useful, but it's a proposal/recommendation.
For instance, I don't leave a space after # in an inline comment in order to fit in 80 characters. The bot will notice this and put the space, therefore 80 chars are exceeded, and the bot will break the comment or put it into a separate line, which decreases readability. Or how does it work at all?