On my tri bike I went for a fit. I don't know that it changed my position much, but I wanted to see the process first-hand. It was fairly comprehensive with markers attached to my knees, ankles, hips etc, and some cameras and software doing some of the observation. In the end we shimmed one of my cleats a bit, which seemed to improve how my knees tracked. Raised my bars slightly and adjusted the fore-aft and tilt of my saddle slightly. My frontal area was reduced minimally, and my power output was raised a bit, and my comfort noticeably. The big benefit for me was the measurements provided at the end. When I travel to race, it's really easy to replicate my position whether I dismantle my bike or use a rental.
For my road bike we started by having me on one of those adjustable bike rigs. The measurements were used to get a Moots built for me from scratch.
Both of these were mostly intended to satisfy my curiosity. Avid cyclist or not, I would suggest that a fit or a custom bike is near the end of the discretionary spending scale. Given my life situation today, I wouldn't spend the money on either. When I ride, I never regret the decisions I made then.