There is some overhead with processing edit submissions. Note that the rule does not apply, when you fix your own post without needing anyone else to approve your edit.
Where the balance between the cost of edit review and the benefit of the edit actually happening lies, and whether a mechanical rule can approximate it, is of course somewhat controversial, but for the time being it is the 6 character rule.
The rule tries to ensure that when you spot a typo and go fix it without much thinking, you should take the same opportunity to revise the whole post, as opposed to submitting all typo fixes into the edit queue as you are discovering them one by one.
If you think that there is nothing else to fix in the post, and that the single letter fix is essential, really absolutely necessary to make the post clear enough, feel free to any of the following, in the increasing order of how much fuss politically you want to make about that kind of typo, and about the 6 character rule.
- Throw in an additional small edit that does not make the post any worse.
- Use a comment to ask the post owner to do the fix themselves.
- Flag for moderator attention.
- Raise a support case on meta (question on meta tagged with support)
Expect that if you involve experienced users, they will typically find other issues (formatting, style, grammar, title searchability, tags) in the post and they will look at your request as something that you could have done as well.
Finally, note that the new edit queues (just launched) are expected to make processing of the edit queue much easier for approvers and they might make the need for the 6 character rule eventually obsolete.