Free DLC and add-ons
Extra content for games you already own. Every item here needs the base game installed first, which is exactly why it is not mixed in with the free games.
You need the base game
DLC is an add-on: skins, weapons, maps, characters or expansions that attach to a game you already have. Claiming it without the base game does nothing useful, so check what it belongs to first.
Why these are separated
Most free-game lists dump DLC in with full games, so you click something promising and find an item pack for a game you have never installed. Keeping them apart means the main list stays honest.