If you are writing nonfiction and pursuing traditional publishing, the answer is usually yes. Not all agents require a book proposal, but it is generally the most effective way to present your work, whether or not the manuscript is complete.
Most literary agents and publishers prefer a book proposal because it allows them to quickly evaluate a project’s market potential. It also gives you the opportunity to demonstrate not only that you can write a strong book, but that you understand the key questions behind any acquisitions decision: Who is the audience for this book? What platform or reach does the author bring? And how does this book fit within the current marketplace of comparable titles?