Who mixes in mono any more? Well, virtually no one, and in most cases you don’t want to end up with a final mix in mono. But if you start mixing with all channels panned to center, it’s easier to hear which tracks “step on” each other; you might not notice these problems if the tracks are separated spatially with panning.
For example, guitar and piano can occupy similar regions of the frequency spectrum. When panned in stereo, they’ll definitely sound separate but in mono, they might interfere with each other. In this case, EQ can help differentiate them—for example, add some low end to the piano and warm up its midrange a bit, while gently reducing lows on the guitar and adding some extra brightness. (Or, do the reverse if you want the piano to be more prominent, and the guitar more in the background.)
If you work with your tracks so that they all sound distinct in mono, when you start panning them and creating stereo placement, the sound will really open up. You’ll hear a sense of spaciousness and clarity that can really benefit the overall mix.
So next time you set up a for a mix, try panning to center first . . . you might be surprised at how quickly this identifies “problem areas,” which you can then fix before proceeding further in the mixing process.