Short answer: it depends. If you have live rock in the tank, then they are unnecessary. Some people use them, some don't. Some will suggest that they are "nitrate factories"... they don't create nitrate out of "nowhere", but they do not help convert nitrate into nitrogen gas. Some folks never have an issue with them, others find it's easier to maintain lower nitrate levels without them - your mileage may vary.
If you do not have live rock or other significant biological media, then DO leave them in place, that is where the aerobic bacteria will colonize that will convert ammonia and nitrite into nitrate.
Jenn