Mono compatibility matters because many playback systems are effectively mono: phone speakers, laptop speakers, Bluetooth speakers, club PA systems, and Amazon Echo/Google Home devices. A mix that sounds great in stereo but collapses to mush in mono has phase issues.
Checking in mono: most DAWs have a mono button on the master channel. If elements disappear, change volume, or sound phasey when collapsed to mono, you have phase cancellation. Fix: reduce stereo widening effects, center the low end, and check correlation meters. Many professional engineers do 80% of their mixing in mono, only panning at the end.