I find the topic of tuners very fascinating, especially now that they are starting to gain popularity in the PRS/NRL type shooting disciplines.
Personally, I struggle with the purpose of a tuner for PRS/NRL and like disciplines. F-class and benchrest are different disciplines in which I personally have no experience, so I'm not going to comment on that.
If you have a custom gun, with a quality barrel, a chamber optimized for your round, your reloads optimize to your chamber, a tuner is not going to make your gun shoot any better. I say this as someone who's tested this. In benchrest and F-class, the shooters that use tuners recommend that you still do a full proper load development prior to doing any "tuning". They are not using their tuners to make the gun more precise, but rather using tuners to "widen the node" of their precision, to keep the vertical of their loads as minimal as possible as the environmentals change throughout the match. Unlike PRS and NRL, in these disciplines every little difference in precision can make or break you at a match. But tuners are also a bit of a dark art, and I even saw an argument occur between a top tuner maker and a multiple world record holder BR shooter, on how to use a tuner. So even in BR and F-class they can't agree on how to use them.
Now, with that said, I've heard a few anecdotal accounts that a tuner has the potential to optimize how factory ammo shoots in your rifle. Once you have it, you can't change the dimensions of your chamber, and you can't change the bullet seating depth of factory ammo without any major intervention, and this is where tuners may help for these disciplines. Some pretty experienced shooters swear that it works, and having not tested this for myself, I'll take their word for it.
Keep in mind, that adding a tuner to the end of the barrel is another layer of potential problems. It's a device that's designed to change your group size and POI, so if it shifts by accident during a match, it's going to cause you issues. It's a single point of failure. Yes, any device that's attached to the muzzle and comes loose will have negative impact downrange, but there's another layer or element added to that with a tuner. Also, if you don't have a good grasp of internal and external ballistics, and how your ammo is influenced from the chamber to the target, adding a tuner is another variable which may add confusion and potentially lead you down the wrong rabbit holes if what you are seeing downrange isn't adding up. Just something to keep in mind.
I'm really interested to hear more reports about tuners as they become more and more popular in the "practical" shooting disciplines. If they do in fact improve precision with factory ammo, or reloads you just can't get to work, I think that would be pretty fascinating and exciting. But I think people need to also realize any potential pitfalls with adding a tuner to their system that's being used in a sport where we beat our rifles up.