What are the advantages and disadvantages of sub-contracting of work in a project?

Advantages 

1. Project manager can hire a subcontractor when he needs. This provides more flexibility of fast track projects
2. Project manager can use a subcontractor for one-off jobs and jobs requiring specialist expertise or fast turnaround
3. Subcontractors can use for small tasks which consuming more manpower and main contractor can concern in core aspect of the project
4. Permanent staff of main contractor can concentrate on major activities of the project
5. Some Sub Contractors can start work at short notice, even when large numbers of workers required such as binding of reinforcements, formwork, pouring concrete etc.
6. No need to worry about insurance for labours and equipment

Disadvantages 

1. Subcontractors may cost to the main contractor than planned value of the project
2. Main contractor will not acquire skills or develop skills of his staff because of outsourcing
3. Because of subcontractor involvement the contract value may raise high
4. Main Contractor may not have direct control over the quality of subcontractor’s work
5. May lack the motivation and commitment of permanent staff of main contractor