Von Eigenes Werk, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=1923693
Consider the following setup. You provide space to store containers and your customers have, after bringing the containers to you, the flexibility to retrieve them whenever they like. In order to not waste space you stack containers onto each other leaving you with a fundamental question: which container should be on top and which at the bottom? It is clear that you want „fast“ containers, i.e. containers that will probably be retrieved in near future, should not be stored under a „slow“ container, because then you will have to rearrange the containers in order to get access to the fast one. The key question hence is, how to find out in advance, which containers will be slow and fast.