Since a Stack Exchange website is not a forum, the comments section is the one place where members can interact in an on-topic manner (I know that for more general interactions we have the chat section.)
Therefore users, in addition to making good use of the comments section for all the intended purposes, will also appreciate or disagree with the post owner, express thanks and engage in broadly relevant discussions in the comments section.
I personally feel it entirely justified to post constructive comments identifying a very good answer, quoting a short extract of what I considered its best content. When I am the person asking the question I will do the same but add sincere thanks for the kind members making the effort to answer my question.
This is a more specific way to draw attention to the qualities of the post, in addition to generic upvoting; and also my way of appreciating and encouraging members who are doing some very good work here. It may or may not come within the strict definition of comments, but it cannot be considered irrelevant by any decent standards, and I am happy to note that my such comments have been upvoted and not usually deleted.
I have noted that the type of less relevant comments you refer to are posted more often by new users who are probably not fully aware of the site policy regarding comments, and less often by experienced users. It is tolerated by moderators and the community as long as the comments are not blatantly off-topic. Moreover comments are ephemeral and less relevant comments will be deleted by the user or by a moderator (whether or not flagged by another user) sooner or later, giving a tidy and 'serious' look to the comments section.
There is also a graded system of 1 week --> 1 month (repeat offence) --> 1 year suspension for users who persistently misuse the comments section for posting off-topic matters or for conducting prolonged discussions/ arguments even after being warned by moderators.
So yes, we are indeed strict about insisting on appropriate use of the comments section!