Using mono monitoring frequently when mixing or mastering could show off many flaws either for individual tracks or the master bus. It is important to check if there are any phase issues and fix them if needed. It is critical to have in mind that many end listener devices may use mono system when playback.
Those issues are result of cancellation and are more detectable when listening to mono. Sometimes we may like the resulting effect and use it creatively.
But what happens when it is a problem?
We can address partial phase cancellation which usually results to volume reduction and unwanted decreased attack or hollow sound of the signal and complete phase cancellation which leads to the total disappearance of the signal. When listening to mono those issues are fully exposed so it is a great way to address them.
I also like to make a lot of equalizer adjustments to the individual tracks in mono, as it is more helpful to detect frequency masking issues. When mixing in stereo some masking issues could not be exposed to our ear. E.g. we have two instruments which one is hard panned left and the other hard panned right and they sound nice. But then when we listen to a mono playback device we feel that we cannot distinguish them. Equalizing them in mono we could lead to more efficient decisions so each one takes its seat in the mix.
I could not tell in which percentage I mix / master in mono and in which in stereo. It is all about each different project characteristics and challenges. The goal is to achieve the best possible sounding result for both stereo and mono playback devices. So I often turn to mono and stereo during the whole mixing and mastering process.