They carry the schedule, the subcontractors, the permits, the inspections, and the liability. The fee, typically 10 to 20 percent, buys coordination and someone who is responsible when a trade does not show or the work fails inspection.
More detail
The fee looks like a markup on other people's work until you price what happens without it. What the role covers. Scheduling trades in the right sequence so the electrician is not there before the framing. Holding the permits and being the party responsible at inspection. Carrying general liability and workers compensation so an injury on your property is their insurer's problem rather than your homeowners policy's. Managing material procurement and lead times. Handling the trade that does not show up, which is the largest hidden value and the least visible. And standing behind the work afterward, which means having one number to call when something fails in month eight rather than an argument about which trade caused it. Typical fee in this market runs 10 to 20 percent of project cost, higher on smaller and more complex jobs and lower on large straightforward ones. On acting as your own general contractor. It is legal for a homeowner, and it saves the fee. What it costs is your time in large quantities during working hours, your credit for material purchases, and your liability if an uninsured worker is injured. You also lose trade pricing and priority: subs schedule their regular general contractors first and a one-time homeowner last, which is why owner-built projects routinely run twice the duration. And you become the party responsible at inspection, with no experience of what that jurisdiction expects. Where self-managing genuinely makes sense. Single-trade projects with no sequencing, such as a roof replacement or a fence, where hiring the trade directly is straightforward and there is nothing to coordinate. Where it usually does not. Anything with three or more trades in sequence, anything structural, and anything on a deadline.