Board Thread:LOC General Discussion/@comment-98.247.145.71-20130817011002/@comment-5706972-20130820135309

It basically works like an 'inactive inventory'. You can store cards in it, but they will not be used in your hand. When you take it out of the box, it goes back to your inventory for use.

Basically, if you have a card that you enjoy looking at, but don't want in your hand; or you want to clear some space in your deck during Raid/Odyssey; etc.

I recommend you buy the mini one to start, and then see if it is worth 2pp per card for the others (equivalent of selling those rares on MB).