The ban on edits that change only a few characters can be quite annoying in some cases. I don't have a problem with trying to keep trivial spelling corrections and such out of the peer review queue, but sometimes a small edit is semantically significant. For example, it makes perfect sense to edit an otherwise good question/answer to change a $0$ to a $1$ or to change $a\implies b\implies c$ to $(a\implies b)\implies c$.
In most cases, if there is one error to fix there are more of them in the post and you should fix them all. Each small edit bumps the post to the frontpage, too many unnecessary edits are harmful due to that.
The minimum of 6 character edits is only active for suggested edits, which additionally require review by other community members. These edits have an even higher cost due to the necessary review by other users. Of course this rule might sometimes prevent a useful edit, but it is a trade off against the potential unnecessary bumping and additional reviews necessary for those small edits.
If you see a small mistake you want to fix, look for other problems with the post you could fix. And once you have 2k reputation you can fix those significant mistakes that only require a few characters to change.
There is no absolute ban on small edits. It's up to each person who has enough rep to edit to decide whether a change should be made.
The main reason is to avoid small changes is that an edit can bump a question onto the front page. This can be particularly problematic if someone edits a few dozen questions in a row. So for "nitpicking" changes, particularly to older questions, it may be better to just let it be, if there is little chance of confusion. It is less likely for someone to make large numbers of substantial edits quickly, because these require more thought.
Another reason to avoid small changes it that the person who wrote the question might feel irritable about having someone point a trivial mistake, just as a speaker may be irritable if someone stops the presentation to point out a trivial typo on their slides.
In both cases, the question to think about is whether the benefit of the edit outweighs any potential negative effects. If there is a substantial benefit, there is no reason to avoid the edit. If there is little benefit, particularly for older questions, then the edit might be better if not done. You can always leave a comment instead.