The general contractor is responsible for the building as a whole: from preparing the site to handing it over.
Coordination
The client has one counterpart instead of a dozen crews. Structure, services and finishes are settled inside the team, not at the joints between contractors.
The most expensive mistakes on a site happen not inside a trade, but at the edge between trades.
Documentation
As-built documents are prepared alongside the works, so acceptance does not become a separate stage of several months.