Eq balance - when in mono, there's no hiding sounds in the side information. If there is overlap, its much easier to spot and hear the mistake.
Phase issues - phase cancellation occurs when you sum the l&r channels to mono and their waveforms go in opposite directions. This causes cancellation, loss of volume, and a "Chorus" sound. Mono lets you spot these negative side effects.
Strong center of mix - you can have both a wide mix and strong «mid" channel. This often makes the best sounding mixes. You want to make sure your main elements still show up in mono and retain their sound quality.
Volume balance - phase cancellation will cause different volume levels when listening to stereo sounds in mono. It's still very important to make sure they fit in with the rest of your full, stereo mix.
Playback devices - your stereo mix is so important because majority of listening devices are stereo. Headphones, car, Laptop, etc. It's not worth completely sacrificing a stereo mix for mono compatibility. Club systems can be mono, but in the end..Balance is key!
Audio Mixing Mastering recommends - reference and mix in both mono and stereo. Also take into consideration where your track is intended to be played…Club? Mono mix might be more important. Streaming/radio? Your stereo mix, etc.
