So, why is MONO important? NOT!

Mono is a great tool to have under control.  Your music ends up in stereo for sure but mono work earlier leads to better final stereo images.  I prefer the mono speaker to be a single speaker in this case and separate to the main speakers. Mono sounds better to me being heard from one speaker alone.
 
It can be used in a technical way to check mono compatibility with say a hardware synth sound that has used excessive phase devices in its final effects processing.  I once had a hardware synth patch that sounded amazing in stereo and collapsed to a heap in mono.  Loss of level and terrible tone change.  In the end I shifted the phase of one channel by 90 degrees I think and narrowed down the image.  It still sounded stellar wide in stereo but collapsed very nicely in mono not loosing level now and sounding robust.  It means it will always sound wide but if for any reason the stereo image is collapsed to mono the synth part will stay behind and remain.  Good for checking drum buses too for solid mono sound but just nice wide stereo sound.
 
One artistic creative device might be jumping from a full wide stereo image to a total mono image.  For a short time.  eg 4 bars.  Getting the mix to sound good in mono takes some practice too.  It forces you to make more changes to similar sounding parts to separate themselves.  Back in stereo once panned etc they sound clearer then.  Turn them down now a little and they are still heard.  Maximum illusion minimum voltage.
 
Another creative effect might be jumping from the normal stereo mix to a mono mix again but panned to one side with the out of phase copy on the right side.  ie in phase L and out of phase R.  When you use this effect you might want to temporarily edit the EQ of both sides to make it sound better for the duration.
 
Listening to your mix summed in mono on a small speaker at low volume tells you a lot too. If there are too many things going on it will be revealed in this check.  All the parts are lined up with each other in the same spot.  It clutters everything.  After pealing back some parts and simplifying tracks everything will sound clearer in mono.  Just the right amount of things going on.  Back in stereo and panned it will sound twice as good.
 
It is also good to listen to just the left channel only to make sure the vocals (or lead instruments) and important stuff is still present in the mix.  And same for right channel.  Listen to what is just there.  Many times I have been in cafes or restaurants and there are two speakers set up but only one is working.  I have composed music for AM radio and in many TV soundtrack situations where my music ended up being summed L+R for whatever reason.  I got caught out badly once in that situation.  (A very important part nearly disappeared because I was being smart!)  It is good to know how that is going to sound by doing a mono sum check.