Putting a backpack down a table, can make it easier to modify them or to keep them in a aesthetical and visible way
How to put a backpack down?
Just drop it on one of the following blocks
Crafting Table
Loom
Cartography Table
How to get it back?
Since 1.21, just click on it
Otherwise, shift while pointing at it.
What can I do when my backpack is "benched"? (1.21+)
Since 1.21, you can click on it while holding a dye item to color it.
Also, you can use a mace to convert old backpacks from 1.20 and previous versions to a new one and retrieve the items inside it.
Most of the new features about backpacks will involve the use of benched backpacks. Stay tuned.