I usually only fly caps, but I usually only do missions and go around doing reconnaissance missions. I'm a loner zoner.
I fly them so I can do cap ship missions and so that I cannot be easily pirated.
My mentality when I started wasn't really, "I'm going to get the biggest ship in the game". It was, "wow trading is really boring, I wonder if I can make more money doing cap ship missions." Sure enough, it's true. I can easily make 60-80mil/hr doing cap ship missions.
I don't want to join factions for a couple reasons. I don't like drama and I generally like playing alone. I'm a loner in life, so I'm a loner in games.