Well, the specific use I had was to have a simple input into machines. When you stack them you don't need to whack them to make the tank work.
Wait, that reminds me, Peppe's examples aren't entirely accurate. If you're inputting with a liquiduct you don't have to whack the liquiduct, whack the tank. See how the bottom is blue? That indicates it is an input. If you whack the tank, the bottom turns orange and it outputs. No need for a redstone signal.
So, getting back to what I use them fore. I used to drop them on top of my machines, then whack the tank to empty into the machine. However once I got mats for several of them what I did was have one attached to the machine at all times. When it was empty I would take the full one, place it on top of the empty one. The full one would recognize it is on another tank and automatically turn orange, filling the empty one. No need for the wrench to whack it to change modes.
Yes, I'm that lazy.
Hm, video idea. I'll put together some examples and post it here in a few hours.