The Downloadable Content Object is a scriptable object created by the Steam Settings when you import DLC. This can be used to check if the local user owns a given DLC and to perfrom common actions against that DLC.
Most modern games do not actually download content with DLC rather they are usually only used as license to check if given content in the game should be unlocked.
This is in particular important for multiplayer games where you need all players to have all content in order to play together but want to lock/unlock content based on ownership of DLC aka expansions or modules.
It is however possible to assoceate a depot with DLC and have it actually download additional game content.