Manufacturers don't make as much money when you reuse the filters, which are actually full of beneficial bacteria.
From Lori Barile:
http://www.aquariumadvice.com/guide-to-starting-a-freshwater-aquarium/
"Filtration is very important for any aquarium and the more filtration the better. You generally want a higher level of filtration than your tank size; so if you have a 30 gal tank and your filter is rated for aquariums up to 30 gals you may want to invest in a larger filter (or more than one). A good rule of thumb is purchase filtration that is double the size for your tank, so if you have a 20 gal tank purchase a filter that’s rated for at least a 40 gal aquarium.
There are many types of filters: under-gravel (UG), hang on the back (HOB), internal and canister. Most internal and HOB filters are the best choice; canister filters can also be great for larger tanks. Some recommended brands are Aquaclear (HOB) and Fluval (internal or canister).
Most filters come with filter media which is the “stuff” that goes inside the filter; this can be ceramic rings, sponges, pads, etc. Some filters also come with charcoal. There is some debate on whether charcoal is needed for an aquarium. Mostly charcoal is used to remove medications from the water after treating for illness but generally charcoal isn’t needed at any other time. You can keep the charcoal in the filter without any harmful effects but you can also choose to replace the carbon with another form of filter insert such as a sponge or ceramic rings.
Most manufacturers say to replace filter media every x amount of months but this is incorrect. The beneficial bacteria your aquarium needs mostly live on your filter media and replacing the media can cause your tank to recycle depending upon the amount you change. Generally filter media does not need to be replaced until it is literally falling apart (which can be years) and if you do need to replace some filter media replace a small portion at a time."