A lot of posts in here but I'll try to bring some clarity. There are regular old cleanings and they are deep cleanings. Deep cleanings are tied to a diagnosis of periodontitis while regular cleanings are linked to a diagnosis of gingivitis. The difference between the two is that in periodontitis there is destruction of bone and other important tissues that connect your gums to your teeth.
Now, usually with periodontitis there is a build of calculus sub-gingivaly (the area between your teeth and under the gums. What the dentist or hygienist will do is go that area and make it scrape up the calculus (hardened plaque) and diseased tooth tissue. This is important because as long as both of these things stay there, you will continue to have bone loss. The reason being because your body's immune system is waging a war against these two things and destroying your own tissue in the process. What will happen is the dentist or hygienist will get you numb and then proceed to an Sc/RP (Scaling and root planing). Going forward I recommend to brush twice a day, floss and go in for constant check-ups because once you have periodontitis you are always at risk for it re-occuring.
Now let's say you don't do that. What will happen. Well people lost teeth mainly 3 ways. Trauma, cavities that make the tooth unrestorable and periodontal disease. Focusing on periodontal disease, what happens is that the tissues and structures in your mouth become so broken down that your teeth become very loose. And eventually will have to be extracted if you want to have a mouth that is healthy.
I've had patient's come in where I could literally just extract 10-14 teeth in like 5 minutes because hey were so loose. You don't want this to be you.
This is a good example of a healthy mouth. Do you notice the height of the bone?
This is an example of an unhealthy mouth. Look at the bone loss. Also look at the sides of the teeth. The gif has arrows that point to something called calculus. This is what the deep cleaning is for. They will go and remove that.