In this method, contactor will bring all the materials for your building construction and also contractor will manage all the daily labors work also. You need to have an agreement with him. He will monitor your building work on behalf of you.
Contactor will be one point of contact for you. You no need to hire/negotiate with any workers and with shops to get material.
You will not have a chance to choose any workers and all of them will be managed by contractor.
There is a good chance of using second quality material for construction where ever is possible. This is one of the area all the owners will have major concern with this method. To mitigate this problem one need to have proper quotation from contractor for each item.
Since it is not his work contractor may not concentrate more on this work. He may do the work with usual attention not with at most care.