Building your own pc is a great experience, it'll let you learn about the internals of a pc and how they go together.
Plus, you get to pick all the components you like, and don't have to settle for some cheap optical drive or memory like some computer manufacturers use. Totally customizable, build it your own way.
Plus the parts are aftermarket parts and you can buy replacements right off of Newegg or any other site that sells pc parts. Unlike a Dell or Gateway or any other premade computer, when certain things go bad, you have to buy it straight from them and pay an arm and a leg for it.