middle click reload your stack

When you middle click on a block, if it's already in your hotbar, in addition to changing your slot to the one containing the block, it reloads the stack.

(ex: if you have 3 cobble in your hotbar and 32 left in your inventory, instead of opening your inventory to reload the stack, you can just place and middle click the cobble to have it reload to 35, it can be useful for building and for pvp (manhunt and smps for example))