I could see how if you have been riding for many years, you might not benefit from a fit but I was very pleased with how mine went. When I got my new bike I rode it stock for about 500 miles on it. The bike I was riding previously was really a size too big for me so just getting the right frame size was a big improvement and I thought it was dialed in pretty well with just a saddle position adjustment I did.
When I got my fit, the adjustments were pretty minor but things I may never have figured out. The fitter noticed I seemed to have more pressure on one arm when on the hoods (likely due to damage caused by motorcycle crash years ago) and moved the left shifter down 2mm and it was quite noticeable. He also noticed my right knee angling in at the top of the stroke when spinning and put a small wedge under my cleat. Other than that, he added 10mm to the stem length and adjusted my seat height and fore/aft position maybe 1-2mm each.
I really noticed the difference after the fit and have no regrets. For us relative novices who don't have years of experience trying different adjustments, having a professional check your fit can be well worthwhile.