I found [.] in a regular expression on manpage of notmuch:
notmuch search 'from:"/bob@.*[.]example[.]com/"'
It seemed to be useless because brackets are for list but have only one character, but finally I learned it matches a literal dot.
Then, why they use it rather than \.? Are there any advantages on this expression?
At first I thought that this is to avoid double escaping but on further consideration I think this is because a dot in a character set ([]) is treated differently than normal. It makes sense that in a character set a dot only matches a literal dot, the whole point is to match a specific set of characters so having a wildcard in the set doesn't make sense.
So [.,;:] may be used to match punctuation marks.
Once you take that into account it's obvious that [.] just matches dot.
Whether to use \. or [.] is left as an aesthetic decision.