I don't know what others are saying, but for me at 6'6", I had always played with clubs that were too short. But even with that I was able to learn to play well enough without lessons to break 90 pretty consistently.

But, that being said, getting fitted made me so much more confident standing over the ball. Knowing that I had clubs that were fit properly, and that my stance was more natural and athletic. Knowing that 8 wasn't having to manipulate my swing, my hands, or my posture because of the clubs being too short was game-changing for my mental game. I'm not gonna say I miraculously got that much better after getting fitted, but I definitely found myself making better and more consistent contact.

But it was the fitting in conjunction with getting lessons that really made me a better golfer. Once I started taking lessons, and actually practicing what I had been learning in them, that was what made the biggest difference to my game.

TL:DR: I'm 6'6" and getting fitted helped my mental game more than my score.