It's an important part of being able to mod the game, both for mod creators and people looking to install mods.
(Probably they now regret calling it OpenIV). OpenIV is a tool for browsing and editing base files for GTAIV, Max Payne and now GTAV. First up: mod tool OpenIV has been updated to support Grand Theft Auto V.
GTAV is similarly devoid of official modding support, but it seems likely that people will again prove undeterred. Despite no official tools to support it, Grand Theft Auto 4 had one of the best modding scenes around, adding new cars and improved textures, rocket punches and gravity guns, letting you play as a police officer or Superman, and much more.