If you work in a Human Resources/Personnel department at a large corporation, or work for a small agency that sells temporary labor or executive search services, then you're in the business of evaluating personnel needs and pitching people and their skills. You may need to convince your boss or a new client of the need to create one or more new job positions, or persuade the boss or client to fill existing positions with personnel you recommend. Perhaps you are persuasive enough to do that with a phone call or casual conversation in the hallway, but odds are better that you will need to write a proposal to pitch your ideas and persuade the client or upper management.
It's all about the relationship first. Many newbie business owners give a proposal to anyone that they think seems interested in their services. That is an easy way to waste your time. Before you submit a proposal, you should have already developed a strong relationship with the buyer and showed the value that you are going to deliver. Your proposal should be a reiteration of that value and relationship and should only be submitted when you know that you are going to make the sale.