It allows you to lock in land, labour and construction costs. In a rising housing market this means that by the time the property is built, it could be worth significantly more than what you initially paid for it.
Because both, the house and the land it will sit on are sold as one package, the purchase process is more streamlined. All the expenses involved in buying the land and building the home are combined in a single loan from your lender. Having a single financial arrangement will often mean lower arrangement fees.