This is just about road bike gearing. For now.
Double clangers have traditionally been 52/42, sometimes 53/39. Fine if you ride in the peleton at 45kmh but rather highly geared for lone mortals.
The introduction of triple chainsets was a (successful) attempt to provide a wider range of gears on road bikes, however, purist anoraky fukcers like me can't look at them without wincing; THEY JUST DON'T LOOK RIGHT! The rear mech is too long etc etc. I have no objection to them on principle, each to their own, bums on seats and all that. I just don't like triples, I don't even use one on my Mountain bike, sacrificing a bit of top end and preferring the slick shifts of a double 30/40 set-up.
Compact 50/36 is an attempt to provide a more useable set of ratios for non super heroes but which still retains the benefits of a double ring set-up, light weight, short cage etc.