Siege operators load and fire siege engines. Higher skill levels improve the firing rate and accuracy, but exactly to what extent is not clear.[Verify]
Each shot from a catapult earns the operator 60 experience points (30 to load, 30 to fire), or 300 total stones flung to become legendary.
The main problem with this job is that, since it is not a military job, dwarves operating the devices are civilians and so will frequently be scared away by threats that they're supposed to be firing at. If they have certain hauling labors turned on, they also will leave their post to participate in corpse-looting, which usually results in a horrible death. More importantly, the unmanned equipment will consistently fail to fire on its own.
See siege weapon for a discussion of planning and placement.