The point of seafoam is to actually remove those carbon deposits. In and around the combustion chamber, including the valves, tops of the pistons and even within the rings and their grooves.
It additionally cleans the injectors somewhat, causing the car to immediately start and idle better (that is, after you get the car started after the initial treatment). Excessive carbon deposits have a whole host of bad things associated with them some of which are the ability to actually change your compression ratio, create hot spots in the combustion chamber, raising combustion chamber temps in general, and jamming the actions of the valves and piston rings. Although I know it say's you can add it to your crankcase, I would strongly caution against using it this way because it has the ability to rinse the rod and main bearings on older engines to the point of causing them to let loose and create the dreaded "spun bearing" situation. I've personally seen that happen twice!