The amount of frames you actually see is limited to the refresh rate of your monitor. If your monitor has a refresh rate of 60Hz, having V-sync on (Adaptive or not), can lessen the strain on your GPU core, resulting in lower temperatures and power usage. Given that your GPU is capable of sustaining a steady 120FPS on a 120Hz monitor, the picture will look much more fluid. Our eyes are pretty much capable of seeing an infinite amount of frames per second, but it’s how we differentiate them between one another and how we perceive them because of factors such as contrast, illlumination, colour, etc.
Keep in mind though, it’s not only what you see, but it’s also how the game feels.