Carbon is not just for things that may already be in the water used for water changes.
In most situations the bad things that carbon removes should already be removed with a healthy water change schedule. However, there are some situations that could benefit from its constant use. One big example in saltwater is that leather corals give off small amounts of a mild toxin. This can be most evident with SPS
in the tank, which will not do as well. Corals in general carry out chemical warfare or may be triggered to carry out direct warfare (stinger tentacles) by chemical cues in the water. In both cases uses carbon (at least occasionally) could be very beneficial. Some use it all the time, some only use it for a day once a month.
I say try it and see if you can see any difference. If not don't keep wasting your money on it.