GovernorSilver

You buy one based primarily on your expected commute - how long it is, is the terrain flat or has lots of hills, will you be riding primarily on roads or is a good part of your commute on dirt/gravel, your budget, etc. Whereas if you were looking at bikes for a specific sport (trials biking, road racing, mountain biking, cyclocross, etc.), you just buy the bike best suited to that sport.

Of course you could be blissfully ignorant like I was when I started bike commuting and buy a bike unsuited to your unique commute requirements. I had the handlebar and other stuff changed on my first bike to make it more tolerable for a 14-mile commute with some hills, mostly paved roads and trails, but... lessons learned!

I've seen all kinds of bikes in our bike parking room at work - mountain bikes, racing road bikes, touring road bikes, etc. I've even seen a cargo bike.