This is what covers over the back wheel, as you would imagine people tend to use it as a brake to slow down. A brake can be used in that manner although if used incorrectly or too much, it can lead to breaking/wearing down your wheel. (flat spotting) So be gentle on the brake and don’t skid !
The main purpose of the brake is actually to guard your foot from your back wheel! Each deck is very specific in the brake bolt pattern so they’re not very interchangeable & you’ll need the specific one for your scooter deck. They are a replacable item as they do get damaged through normal scooter use, and you can also replace it on some scooters with a wheel guard, which means you don’t have a brake on the scooter, but you are achieving the main purpose which is to stop a foot touching the moving wheel.