As the buyer, if you send a request for proposal, you will receive detailed responses from multiple vendors. Each vendor will indicate how they will tackle your project and meet your requirements. The gives you an opportunity to see how these vendors think and operate. You will have an opportunity to review their proposals side-by-side. For large projects, this is a pretty efficient method by which to approach vendor selection.
Further, the vendors that respond will begin to ask questions to further understand your project. This means that once you make your selection, you will be selecting from among the finalists that have the best understanding of your project needs.