I am a relatively new member of the site and haven't spent enough time to read through rules and meta posts yet to really understand how it all works. So when I see the occasional discussion in the comment section between highly ranked members of the site, I tend to make up my mind intuitively, keeping in mind that it should help the site remain an open forum, where reasonable questions about physics can be asked. Like I said, most people here have probably spent a lot more time thinking carefully about how to run the site and I appreciate that, but this is just how I generally think a forum should be.
Now I really enjoy this site and how it is run for the most part, but there are some questions where I simply can not understand why closing them is any good for the site. In particular the "off-topic" tag seems a bit off sometimes (sorry for the pun), since modern science is hardly ever part of a single subject (The most common overlap is gonna be maths, but increasingly biology etc.).
Now of course one needs it to prevent questions that are nothing to do with physics. My main question here is: when there is a physics component in the question and when in doubt wether it can be answered here, shouldn't the decision go in favor of the question? I am happy to get to know about how the actual rules are intended, but I'd also like to know from an intuitive perspective for a new member of the site.
There are some other supporting factors, e.g. when the question has a good vote reception. The reason I am writing this post is such an example: evolution/gluon question. I can not see at all how this is off-topic and @Nathaniel comments seem quite reasonable in this context. There was a similar discussion on this one, but it luckily stayed open. Unfortunately I can't make a more complete list of these kinds of questions because I can't figure out how to search for closed questions.
I have elaborated why I don't understand this type of off-topic closing of question, now I'd like to return to the title question and ask more generally than "is Math always off-topic" or "is Biophysics always off-topic": What is "closing as off-topic" intended for? And why when in doubt is the decision not necessarily for the question?
One further thing I'd like to mention is that I of course realize that bad questions that pollute the site have to be closed.