You get a clean, verified installation directly from the developers (Facepunch Studios). Workshop Access: You get one-click access to the full Steam Workshop. Automatic Updates:
The biggest feature of GMod 13 is the Workshop. With a cracked version, you lose instant access to thousands of maps, weapons, and addons. You would have to manually find, download, and install the .gma files, which is a tedious process compared to the one-click "Subscribe" button legal users enjoy.
: Recent updates have introduced better water shaders, giving a new look to the iconic "face" of the game, gm_construct