Your time is better spent playing the game, not hunting for corrupted archives on sketchy download sites. Go to Steam, hit Verify, and get back to Los Santos.
If you are a modder, learn to use mod folders (mods folder) so you never have to modify the original x64a.rpf again. Always keep a backup.
A: Because it contains high-resolution textures, 3D models, and audio for the core game environment—everything from police car sirens to building textures.