Idler pulley

I went to go pick up that 93 Eclipse I've been looking at today, and it was running really bad. So before I bought it I told the guy I had to take it to my mechanic to have him look at it. He said that the idler pulley was pretty much shot. The car has 140k miles on it and the pulley has probably never been changed. I know this is something that you change when you do the timing belt.

So my question is what does the idler pulley do, and does it just go out because when I went and test drove the car 3 days ago it seemed fine. Also will driving the car with a bad pulley have a bad effect on the engine. I would just be driving it about 60 miles at about 55 mph and then hopefully fixing it myself when I get it home.