I voted to close this question but now I'm voting to re-open it.
IMO it's a reasonable question - we see very very often that conductors (and also musicians) express themselves facially and bodily in certain peculiar ways during a performance - clearly that has something to do their performance and it is a matter of interest and perhaps of importance: If grimacing somehow helps you perform better, than being embarrassed to do so in front of an audience might hinder your performance, and perhaps allowing yourself to grimace, etc will improve your performance: Playing music can be a highly emotional experience, that can and should inspire your entire body.
As for @DoktorMayhem's contention
It's like asking why some stick their tongue out during difficult passages...
IMO that's also a valid question.
Clearly, something is going on here.
I thought this was a somewhat interesting question - I also thought of BB King's gurning - and as Dr M says, other facial contortions are also often seen!
But this is a site for questions about how to perform and analyse music - so you need to ask the question in a way that engages people from that point of view. If you were to make it clear that you want to talk about ways to display passion that engage the audience, etc... that might be more on-topic here. Again, it comes back to the point I made in another recent meta answer that you seem to be something of a specialist at questions that are very borderline; If it's your aim to get them answered, do the work to make your questions good questions, according to the standards of the site.