What is actually done when it comes to tuning can vary. Personally, I have yet to find a gun that couldn't benefit from tuning. That includes $2000 rifles. Tuning could include anything from fixing whatever known factory flaws or weak point of a particular model or brand to a full blueprinting to simply tuning over the chronograph and making needed adjustments to get the best possible shot string per the customers wants. Every gun has something that can be made better or improved upon. Trigger adjusment is probably another big one that tuners work on alot.
As an example, If I do a full blueprint and tune, the entire gun is stripped down to every last part. Everything is polished, needed improvement are made, trigger is set how the customer wants, barrel work is done, seals upgraded, and finally tuned for a certain pellet.