Flying in coach for short flights (<2.5 hours) never really bothered me, but once I started flying a lot, it started to make a big difference on the long flights if I could relax. also, from a workspace perspective, if you do need to get laptop work done on a plane, it is night and day trying to write something serious in coach versus business/first class.

Internationally, it's all about productivity at the destination. If I'm flying to australia for work, if it's not in business class (or, dare I dream, first class), I would be a mess on arrival. In business class, I hop off the plane and can go straight to work. I might not care as much if I was going there for a long stretch of time, but sitting on a plane for 20-ish hours from new york to australia, then staying someplace for two or three days, and turning back around and spending 20-ish hours on a plane to get home, it makes all the difference.

The other thing is that, as other posters have thrown around, is that once you fly enough with a particular airline, they start granting you (limited) access to first/business class as a sympathy prize of sorts for spending so much time in the air.