huge things (tent sized stuff!) can be knit on a set of 5 or more circ's, using each circ as a DPN! (after all its is a dpn, its just soft in the center!)
if you are knitting a sweater for a guy, (a big guy) you can get a really long circ, ($$$$) or you can use 2 or 3 Circ's as DPN's. Very often its cheaper, and certain its easier to find 2 30 inch long circs than to find 1 60 inch long circ.
unlike normal DPN's that can'r really be uses in set of 3, circs can (you slide the stitch onto cable portion, and it will work fine!)
and since circs of all lengths are easier to find, you can sub a few circ for long DPN's! (or if your really like the rigidity of dpn's there is no reason to stop at 4 or 5, you can knit with 8 or 10! (2 sets) or any number in between!