Same reason they are in freshwater. To take out impurities. I'll try to explain but I may mess up on some technical terms. Just about anything will bond with carbon (electron stuff), so people use carbon so foreign elements like impurities attatch to the carbon. After a few weeks of use you need to replace the carbon because all of the available surface area that can be attached to by impurities will be used, so sometimes people talk about carbon release some of those things back into their tank. So change it out every two weeks or so (i do mine every week).
Carbon also makes the water a lot more clearer - i've never experienced this because i've always run carbon.
You can put them anywhere, IMO place them after the mechanical filtration (so the carbon doesn't get food stuffs stuck in it) and where it would get most flow.
Hope that helps.
