I'd say "plumbing" just widely describes the concept of "using water below the level it came from without drowning eveybody in the process - or just drowning a few selected individuals".
I mean, you can use water to create waterfalls - to make your dwarves happy. You can -or rather need to- use it to treat the injured. You might want to consider using it to move items, to temporarily deny areas, to train swimmers and MAYBE even to put out flaming dwarves, but I'm not sure that's possible, still. Oh, and you can use water to irrigate farms.
Really, get creative, there is no "this is plumbing and this is not". Well there is. A masons workshop isnt plumbing, period. A hammerer beating somebody up, isnt, either, despite the striking similarity of a hammer and a big wrench as well as the resulting splattering of more or less disgusting fluids when something goes wrong.
First, you think of somethign you wanna do with the water, then we can come up with suggestions for how to build it.
One thing I found useful is to know that you can smooth stone at the very map edge and then carve fortifications in it. That will make that tile drain water. (1 tile will probably not be enough, you need more, maybe even spread across a few z levels)
I thought thats the most elegant and non-CPU intensive way to get rid of water underground, apart maybe from draining into an aquifer layer.