I don't even own a set of straight needles anymore. With circulars you can knit flat or in the round whereas with straights you can really only knit flat except for a few special techniques.
Circulars can knit much longer things like a whole sweater or afghan where it would be very hard to do with straights without making it in strips or parts so you have to seam.
They can also be much easier on the hands. I own a few sets of interchangeables as well as extra needles and cables. I like using magic loop for small circular items like hats so I also have a lot of lo g cables.