The answer is annoyingly simple. If you have mountains, big hills or sometimes even difficult overpasses, it's better (much easier) to have a gear that you can drop into. Also that top end gear is handy if you want to hang with the faster than ordinary riders.
On the other hand I can hop on my single speed any time, wearing anything, because I could enclose the whole chain. I also have no expectation of riding it faster than 18-20 or setting personal records downhill so I can enjoy a relaxed ride more often.
There's not much else to it.