To start with the basics, I want to make sure you know the difference between a GC and a subcontractor (Sub). Subs are specialists in a certain trade of the construction industry. Examples of subcontractors are plumbers, electricians, masons, painters, drywallers, framers, finish carpenters, etc.
General Contractors hire and manage subcontractors. Which brings me to the first reason you should hire a GC- We, as GC's have established good working relationships with subs. We know which subs share our work ethic and commitment to quality work. We know which subs can get their part of a job done on time, and have weeded out those who can't. In our case, subs know that we are fair, organized, and pay them on time. All reasons that they enjoy working for us, give us fair prices and strive to make us happy.
Something that is often overlooked by homeowners is the number of subcontractors that need to be involved in a project. Let's take a Kitchen remodel for example. Typically you would need a plumber, an electrician, a drywaller, a carpenter, a cabinet installer, a tile setter, a painter, and someone to install the countertops. That is a lot of people to manage! For things to go smoothly everyone must be scheduled to come in at the right time and in the correct order. All the materials that they need must be there when needed. A GC can get the job done much more quickly than a homeowner simply because he has experience on his side! Ultimately this results in less money spent and less stress for you.
To put it simply, the General Contractor is the manager of your project. The GC hires and oversees all the subcontractors. The GC also makes sure that all of the products and materials needed for the project are there when they need to be. The GC is the only person that the homeowner will need to sign a contract with and pay directly. The GC is then responsible for paying subs and suppliers. It is imperative that the GC be a good communicator because he needs to keep you informed as to what is happening throughout the project, and he also needs to communicate any concerns you may have to the subs and vice-versa.
As in the case of any job that involves multiple people working together, your construction job needs a manager. Hire a professional General Contractor!