To get you to spend money.
However, I use carbon in all of my tanks. But not like you see in typical store bought items. For carbon to be truely effective at significant DOC removal and clarification, the surface area/volume needs to be large. Not a half ounce or two at the top of an airlift tube or what little is in a cartridge in a HOB filter pad.
A water change will less expensively remove the DOC but if source water also has DOC you do not want the water change is helping maintain TDS/buffer and other undesirables from the fish, but your baseline DOC are still there. The carbon does clarify the water excellently.
An ammonia spike is not the issue. When a cycle completes from initial set up, you do not end up at a perfect zero. Some ammonia comes in, one bacteria builds up and converts it. As the ammonia goes away the nitrite bacteria start to die off until growth comes back with the fresh food source.
The bacteria levels are constantly floating around an equilibriumlevel equal to food available. Just as when you slowly stock up your tank on a fishy cycle to let things balance out.
