3 gun competition users also use them for faster follow up shots. Winning and losing can be separated by fractions of seconds at high levels.
You are correct that their biggest advantage is for suppressors.
I read about shooters trying to increase bolt cycle velocity so they could get faster follow-up shots. When you do the math, one needs to have a reaction time faster than 1/30 sec to benefit from a faster than stock cycle time. So, in my economy, the only folks making out are the ones selling lightweight carriers, and all sorts of tweaked small parts to make such a gun cycle reliably.
Regarding Adj manifolds, there is a benefit when using a suppressor- but not as much as some would suppose. The bullet is gone, BCG is beginning to move and the system is bleeding gas to ambient at a fast rate from three points- muzzle of the can, the breech and gas port. The trade off is once you set it for your load, your rifle may not be reliable with any other ammunition. A bit if extra gs bleeding out the breech is not something to die for, hobbyists, well, that’s who all the boutique stuff is made for.