What is idler pulleys purpose

[talonAWD99]

I went to look at a 95 gst eclipse with my friend today and we noticed one of the pulleys had no belt on it. It was the right top one below the timing belt. What is this pulleys purpose and is it going to cause problems down the road?

 

[RotaryRyan]

well, it sounds to me like the a/c compressor belt. in which case you dont need that unless you want a/c. the other two would be alternator and powersteering, which you want there.

 

[talonAWD99]

ok thanks alot