In other ways, a up-stair down stair in pure form is not unlike a ramp when driving a particular direction.
If you had a workshop that only ever outputs one thing to another stockpile, and never needs to intake anything from closer by than the pile behind it with outgoing traffic going one way (sand for example, just get bags and zone), its more than adequate to use one way stairs for alternative vertical routes across spaghetti floorplans as long as the craft dwarf isnt the one actually carrying the materials to slow down efficiency splitting them away from the workshop (workshop profiles to ensure).
Sand comes in, hauler grabs bags, goes up via up-stairs to minecart, deposits, then visits the nearest tavern because this is the civil-district of the fortress as they crossed layers from the industrial district coming up. You could have just put a minecart infront of it, but now you have empty space to cut a nice path on the industrial layer or another adjacent room and dwarves dont have to run to the nearest central traversal stairwell, being much more preferable to run from a place accessed quickly back down.
Othertimes its a informal enforcement of a traffic rule, parallel lanes to stop cramming down a busy central hallway, if there's a high density of dwarves instead of going laterally around it, you have to take the high or low road with single direction access and exit. With a ramp it'd be still chaos as dwarves would whimsically choose to travel down in either direction on any traffic rule potentially rendering it void of use.