It gets used whenever someone doesn't like something for a reason of personal taste and people who don't really know (like your boss) immediate fixate on it and think "well, we must do that then and why is the other guy (you) suggesting that it doesn't apply in this case? Best practises ALWAYS apply surely? I will have a quiet word with him about standards..."
IOW it's a short circuit to not justifying something. If something is best practise you can explain why it suits the current situation and if you can't explain why then it doesn't matter.
Well, sure, but since every decision has tradeoffs, sometimes when arguing for or against a decision, it's easy for managers to side with the person they have more rapport with, who may or may not be correct, rather than decide on technical merits alone.
So, in some cases, it's more beneficial for the company to adopt a "best" practice, than to have engineers engage in arduous discussions, which can cause resentment and further problems within the team.
Needless to say, it's a delicate balance, which is why I wouldn't want to be a manager. :)
IOW it's a short circuit to not justifying something. If something is best practise you can explain why it suits the current situation and if you can't explain why then it doesn't matter.