Validation works similar to Windows' sfc /scannow command.

It will check the integrity of game files to make sure they are in working order.

This can be after you moved Steam to a new computer or a new hard drive (and the games with it), after you experienced a crash of the system, or after an update of a game on Steam that somehow changed it so that it does not start or work properly anymore.

While you could go ahead and remove games from Steam right away to install them anew, validation is often the better solution.

Verifying the Integrity of your Game Cache will fix your install if there are corrupted or missing files in your game.

I know on some other games I have tried it on, steam will end up downloading some files, but apparently not all files are included in this.

Interesting, I've definitely seen this redownload an exe when I've manually patched the exe so it's checksum would be different.

Create the file in the script's directory and add game numbers to it (one per line).

The easiest way to find the number is to right-click a game and select create desktop shortcut.