One line of products is called starters. These are supposed to add beneficial bacteria to a new system. They will not harm the system, but bacteria is added to the system every time a flush occurs, so it is not necessary to add more from a commercial source.
Some products are marketed as supplying nutrients to aid bacterial growth in septic systems. Everything the bacteria need is already present in the sewage- that is why they are there. When excessive amounts of cleaning products are used, such as chlorine bleach and anti-bacterial products, many bacteria are killed when introduced into the septic system. Reducing usage of these cleaners and disinfectants will allow bacteria activity in the system to resume by itself.
Septic system cleaners are sold to clean the pipes and tank. If the product says it does this by enzymes, it is the same as a starter or nutrient supplier. If they act as a degreaser, they will disperse the layer of grease and oil naturally present in the septic tank. As these solids move into the drainfield, they will plug up the system, causing failure. Some of these products are powerful, and may also be dangerous to handle. These will not replace the need to have a tank periodically pumped out, which should be done every one to three years.