Many homes have them. How do they work? General rule is to have them emptied and cleaned when you move in and then every 3 to 5 years. Empty and clean are not the same thing. Clean removes the scum residue as well as the collected solids scraped from the bottom. Are the baffles in good shape? Are you considering extending your driveway or installing that new shed on the nice level ground in the back? Take a look at what the experts tell you.
I never understood them. The Flintstones had some kind of mini-sourous under their sink. We seem to have some small industrial grinder. This is fine but if you have a septic system, you may be adding more stuff to it that it is not designed to process. Try composting instead.