If you are pitching your idea or services to multiple parties, the one thing you do not want to do is send out a general form letter along with a standard brochure or stack of resumes. That sort of 'one size fits all' approach cannot substitute for a real proposal. The goal of a proposal is to persuade the client or boss to endorse your idea and let you do the job. To succeed at convincing them, you need to focus your message to a specific situation, gain their trust and show them that you know what you're talking about and can deliver what they need.
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.