Lots of professional research has been done, and by and large dental professionals agree that it is essential to have a professional cleaning done every 6 months. Using a toothbrush and floss removes much of the buildup (plaque) from your teeth, but it's very difficult to remove all the plaque every time you clean. This plaque that is left behind, after about 10 days, begins to turn in "tartar" or "calculus" which is a hardened version of the buildup that you cannot remove with home care (toothbrush, floss). Your body sees this calculus as an invader somewhat like an autoimmune disease and starts to try to remove it by becoming inflamed and increasing blood flow to the area(s). This is called gingivitis, which literally means "inflammation of the gums." If left untreated, gingivitis can further progress to a disease known as periodontitis. This is where the inflammation of the gums now leads to bone loss of the bone supporting your teeth. There has been a lot of recent research that suggests oral hygiene health (periodontal) and cardiovascular health to be very related to one another.
Other reasons for six month interval professional cleanings and exams include us being able to find fillings that may have fractured and are easily restorable if found quick enough. Over time fillings breakdown and need to be replaced due to actual damage to the filling or due to decay being found between the filling and the tooth. If caught early enough, most times we can just replace the filling (or whatever restoration is on the tooth already) which saves you money. As more tooth structure is lost to a growing amount of decay when left untreated, treatment becomes more expensive and leads to root canals and crowns, etc.