Good contractors can bring piece of mind, knowledge of industry standards and project management expertise. But they also bring tangible, necessary things to the job: a license, insurance and worker's compensation. If you act as the general contractor yourself, you assume liability for injuries and property damage.
Perhaps the most stress relief comes from the fact that the general contractor is responsible for the quality of all the work he or she oversees as part of the contract. If something goes wrong during the construction, it's up to the general contractor to get it fixed. The cost of those repairs comes out of the contract budget, not your pocket.
Knowledge of building codes, appropriate materials, proper construction methods and safety -- both during and after construction -- also is a key resource that a general contractor brings to your project. This knowledge saves you the time and trouble of having to learn when it's too cold to pour concrete, how long caulk should cure before it's painted and why it's absolutely necessary for the plumber to install a P-trap under your new sink. This knowledge also lets a contractor know which subcontractors are reputable and reliable, and which ones no one should ever hire.
Contractors also have one big advantage you don't: They've done it before. From initial planning to final cleanup, contractors manage workflow like conductors manage orchestras. They understand the structure of the composition, they cue different players when it's their turn to take the lead, and they probably have experience with at least a few tools of the trade. If you've done your homework and hired well, your general contractor will keep the work flowing and the crews busy throughout the entire course of the project.
In the end, it's up to you to decide if your job warrants a general contractor or if you can handle to project yourself. But a general contractor could take some of the worry and stress out of the job for you, so when it's done, you can sit back and enjoy the work.