The soaper skill is used for creating soap from tallow and lye at the Alchemist's laboratory. This is one of the least useful roles in the fortress because any dwarf can be assigned the task and yield the same result since soap has no quality modifier; the only difference is faster production. Unlike in later versions, migrants never arrive with soap-making experience, so any attempt to make soap would require training a soaper from scratch; since soap has no actual use other than as a trade good, however, there's generally no point in training a soaper (aside from possibly attracting the Alchemist noble).
Given that the base value of soap is 25☼ multiplied by the [MODVALUE:] of the creature whose fat it is made from, soap can be very valuable. It is also the best way to get value out of megabeast fat (plus, admit it, a bar of Hydra or Dragon soap is just cool - and very valuable).