A mature tank is just that. When your cycle has finished, you have a new tank that it is safe to put some fish into. At that point many other biological processes that happen naturally in a tank are just getting started. As the tank is occupied over time, many of the other biological processes also reach an equilibrium in the tank and changes start to happen less rapidly. At 6 months, many sensitive fish will do fine that would die if exposed to a new tank. At a year almost any of the very sensitive fish can be kept with success. It takes time for the complete ecosystem in your tank to mature and become stable but in the first month you can remove the worst of the poisons, ammonia.
As others have said, once you have an established nitrogen cycle in your tank, adding more miscellaneous additives will do nothing to actually help your fish. It will line the pockets of the local fish store owner so I am sure they will continue to promote them.