They are used if primarily running suppressed. Most people put up with their rifles being overgassed.
I believe the most popular gas adjustment tool is made by KNS.
The other one is DAG-13 from Definitive Arms.
There are other variants to accomplish the same thing, but I am not a fan of how they are mounted. The Gas block is acting as a clamp on the barrel held in place by two screws. I am not saying it is not a good implementation, just saying I don't like it. See Venom Tactical.