Other than the examples listed here, provide three examples of a) home office GCs costs, b) jobsite GCs costs, and c) jobsite

Other than the examples listed here, provide three examples of a) home office GCs costs, b) jobsite GCs costs, and c) jobsite direct costs.

This is not necessarily an ABC question, but what are some of the reasons a GC would choose to employ subcontractors versus performing the work with its own direct craftsmen?