Instead of needing the wind to fill the sail and shape it the cams give the sail shape, even when there is no wind. This means the sail is more stable during gusts and lulls as it won't be changing shape as much. The cams make the sail heavier and harder to rotate during a gybe etc. thus you'll only find them in freerace/race sails.

Here's a shot of one from inside the sail:



Some marketing blurb: http://www.neilpryde.com/sails-2008/sail-technology/ultracam-2.html