Supressors are legit. The hot expanding gassed coming out of the end of the barrel have a rocket effect. This makes guns much more dangerous. A gun without a supressor decreases accuracy which increases the likelihood of collateral damage.
They are easy enough to make. The baffles inside the canister being the hardest part to make. Cash at a machine shop solves the problem, though.
Supressors are not a magic bullet, no pun.
They reduce from 160 db to 130-140 depending on the quality.
They are essential for predator and dangerous game hunting. The baffles forces some of the gas back on itself, negating the rocket effect, gatly reducing recoil, and the rise associated with the typical levar affect of the typical rifle shape. This enables the shooter to get off a second and accurately fired at a charging bear or moose. The suppression of the flash allows predators to not be able to locate the shot, so the predator does not know what to run away from. This is particularly true when bird hunting.
Also, if you can hear a shot without a supressor, chances are you could hear it if one were in use.
Law enforcement rarely needs to conceal their position, or hide their numbers. They rely on their numbers for intimidation. LE actually has little use for a supressor, the civilian has many.