Contractor's Overhead & Profit is used if you are hiring a General Contractor to manage your project.  Your General Contractor will manage hire, manage & schedule your project contractors and need to be compensated for their services.

Generally, a General Contractor will charge 15 to 25% of the Project Value to manage the project.

So a $50,000 project would likely have $10,000 (20%a0 in General Contractor Overhead & Profit.