In this day and age of plug-ins, stereo imagers, and big wide pop sound, almost all the engineers I know mix in complete stereo sound. As such, mixes can get crowded very quickly (especially when using a maximizer or compressor like iZotope.) This causes loud, crowded mixes, and that's just with the beat alone!
Whenever we (as humans) attempt to solve a new problem, we tend to look at the past to see if there were any primitive fixes, and we then expand upon them.
Thus enters mono.
Old studio's did not have the tech, equipment, and the resources that we all have sitting right in our homes today! HOWEVER, I would be the first to say that some of those mixes were beautifully balanced, seemingly mixed by nuanced hands. Mixing your song in mono (at least for the majority of the mix session) allows you to simply your sound. Pulling everything in can show you where there is chaos, and allows you to easily control the sound.
Come back tomorrow as I delve further into mixing in mono sound, with a focus on producers who use electronic instrumentation!