Many mono recordings are completely different mixes from the stereo version and not a simple fold down of the stereo mix to mono. For people listening in mono to a stereo record, that is the equivalent of a simple fold down and the result may not be as good as a dedicated mono mix.
I know people who actually have extremely high end mono systems. For stereo recordings, they do have to listen to a simple combination of the two channels where there is no dedicated mono mix, which leads to some less than ideal results. Also, I have clearly heard advantages to doing that combination of left and right as more than a simple summing circuit or by software from a streaming service. I like when a transformer is used to do the summing, the sound is less dry and I don't care if the altered sound is an artifact of phase shifting or whatever; it always sounds better.