Michael "Mike"’s Answer
There’s also the wide variety of duties requiring different skill sets in order to perform each. You may specialize in one or more, or become a generalist (covering many trades, but not to as great a level as a specific tradesman).
Specializing in a specific trade typically would rate a higher pay rate, but limit your duties to those in that field. Being more of a generalist provides a more varied list of duties, but frequently at a lower pay scale.