IMHO, it helps for 2 things.
1. As mentioned; sonics (frequency ranges and signal strength of various tracks).
It is possible to actually have phase cancelling going on and a MONO mix will make it more evident, let alone bass and bass drum stepping all over each other on certain notes, etc.
B. It's always good to at least know what the mix will sound like in MONO (maybe it will get played over a PA at some unknown location someday). At least you will have improved it .. whether or not it is ideal.

I do not do separate mixes. I'll do my stereo mix, listen to it in MONO (easy as clicking the MONO button on the mixer in PT or RB). If I can improve by doing so, why not? There will always be a happy medium.

Years ago bands/producers experimented with quadraphonic mixes .. but it had to still work in MONO, which was by far more common at the time than now. These were vinyl records.

These days an equivalent is 5.1 etc.

.. plus 'Soundbars', which are essentially a MONO source 'location' with smoke and mirrors to make you think it is something more. By that I mean all speakers are located within inches of each other .. like bolting your stereo speakers together; the sound comes from one location, but you do get multiple 'channels' ..

There are lots of various listening environments where checking the mix in MONO could be a good thing.

Make your sound your own!
.. I do not work here, but the benefits are still awesome