Pretty simple. If you are in a container menu of any type, middle clicking an item moves it to the opposite panel. If you click on a stack in a treasure chest, it moves the whole stack into the first open slot in your inventory. If you click on an inventory item while using a forge, it moves it to the top first, then the bottom. It would make moving items much faster and easier.
Forgive me if this was already suggested. Wasn't sure exactly how to search for something like this.