Plaque that is not brushed away hardens into tartar within days, and no home brushing can remove it. A professional scaling clears this build-up from above and below the gum line, stopping gum disease before it starts.
A cleaning visit is also a check-up: your dentist spots early cavities, gum changes, and other issues while they are small and inexpensive to treat. For most people, every six months is the sweet spot for staying ahead of problems.