Don't have a direct comparison before and after, because it mostly marked a mindset change of mine. The result didn't show until a year or two later.
Before, my mindset was to run this hobby as cheap as possible, trying to push what was achievable with cheap gears. After several years operating under this mode, I had some ups and downs. I was able to kept some considered difficult stuff at the time, like couple maxima clams and anemone. But after I got into sps, and lost them for one reason to another, I decided to crack down on the uncertainties, in order to narrow down the actually problems I need to solve. Getting a PAR meter was one of the first several things I did then.
After that, I stopped mostly on trying out my own ideas and theories on lighting, especially when I found out with the PAR meter that the spread of my lighting was quite uneven. Instead, I just went for some known good solution, and got two Radion XR15. At that time, I didn't have many local reefers or stores in my area, so I didn't have much a chance to experience what good looks like for reef tank lighting. The PAR meter was the only way to educate myself, and eventually convinced myself for the investment.
In hindsight, the most important role the PAR meter play was to convince my stubborn self that I didn't know better than seasoned reefers and high end lighting brings real value, was not just overpriced snake oils.