I have indeed saved leftovers from the last twenty or so installs. I've bought metric nuts (all screws in mount hardware packs are metric) and was able to ship a parts box full of hardware to my overseas project, since the local stores had horribly sparse stock.

If you install all new, then no, you don't need leftover hardware. But I do a lot of restaurant business where TVs get replaced. We need screws that fit the new TV. We're not going to replace every mount just because we don't have screws!