Yeah that designation method works better.
As for uses...
I drive a shaft right next to a handy volcano. Then I set up an iron pump powered by a windmill, up there.
At the bottom I dig a 17x17 holding pool. I dig another 17x17 room on top of the pool and channel a nice pattern of magma workshop positions.
I d-zed the ramps at the bottom of the shaft, which allows me to build a support there. At the top of my original channel I left a one tile floor to walk across. A miner channels out the bottom up stair case and everyone leaves. The last dwarf out channels out that one floor at the top. Now I have a free standing 40 something level cave- in just waiting to happen at the pull of a lever.
That's pretty useful. Or I can just pull the lever, set up a safety pressure plate that detects magma somewhere along the shaft and toggle on the gear assembly that powers my iron pump. The shaft fills with magma and runs along whatever pathway I set up for it. Now instead of having all my forges 40 z-levels away they are right next to my chow-hall. And I didn't have to 'discover' the magma sea and kill a miner in the process.
I also have my miners throw all the ore they dig out way up there down another shaft covered by a little 3x3 bridge one level up from my metalworks so my that smelters can access the huge pile of rock that they didn't have to carry. No mine carts, or much dwarf hauling.
When they run out of ore on the bottom level, I lock the access doors at the bottom and retract the bridge; and dump another load of stone and ore down into the hole at the bottom. Where my masons and metalsmiths can make use of it without having to run all over the map.
Handy, instead of clearing alot of horizontal space, you acheive the same effect by driving one small shaft through many z-levels right where you most want all your rock and ore to be.