It's pretty simple. Games run at faster frame rates at higher settings with it. The practical outcome is that for any given AMD GPU, you can get the same quality by going a full step down with NVIDIA. Sure, my 6700 XT is not exactly stabbing my eyes out, but a 3060 12 GB was $100 cheaper and could run everything just as well by using DLSS. Example, Cyberpunk 2077 runs at 1440p Medium @ 78 fps on a 3060 12 GB with DLSS, while on a 6700 XT, it actually runs a bit slower, closer to 70 fps.
Is Cyberpunk 2077 perfectly fine at 70 fps? Of course it is. But you I paid $100 more and run my computer hotter for the experience.
The only reason to turn DLSS off completely is you feel moral outrage at knowing some percentage of the pixels are inferenced rather than rasterized. Because like
@Susanna said, the only way you can tell it's on is to take screenshots and pixel-hunt.