From the Portal Wiki
Oh well. We have warehouses FULL of the things. Absolutely worthless. I'm happy to get rid of them.
GLaDOS describing Weighted Companion Cubes

The backpack is a player's personal inventory for in-game items. Introduced in Portal 2, the backpack stores all of items that a player has obtained in individual slots, and allows them to be viewed, moved or deleted from within the backpack interface. Items are displayed in six 50-slot pages for a maximum of 300 items. The backpack enables players to customize the robots Atlas and P-body for co-op gameplay with a range of items, including miscellaneous items, hats, skins and gestures.


The loadout screen shows previews of equipped Miscellaneous items, Hats, Skins, and Gestures for P-body and Atlas, and allows the player to select new items to attach to either robot.


See also