After giving the subject - ooh, five minutes thought, I've come up with the following. Naturally I've excluded anything to do with telling the time.
Status. The worst possible reason for wearing a watch. Nothing else needs to be said.
Image. Often related to status, but not necessarily related to wealth or position. If you're not sure people see you as you want to be seen, get a watch that identifies you as a man of action, cultivated aesthete, rational pragmatist, etc. The second worst reason for wearing a watch.
Technical appreciation. You just love the way things work or the way things are made, and want an example of ingenuity or craftsmanship (literally) close to hand. A perfectly acceptable reason for wearing a watch, but carries with it the risk of pedantic obsession. Communicate your passion and people may avoid you for fear of losing the will to live.
Distraction and focus object. My mind is full of bothersome stuff and I need some kind of release. I'll look at my watch. My mind is empty and nothing is catching my attention right now. I'll look at my watch. A natural impulse during the course of a day, but one that can be expanded to fill a large part of your life. You go from looking at your watch to looking at all the watches in the world.
And why watches? Because they're always there, and wearing them requires no justification. They're the worry beads, the decorated sword hilt, the netski of the modern world.