For consultants or others selling professional services, what often happens, for example, is that you quote a daily fee of £200 or £2,000 and the buyer has nothing to measure it against. How do they know that this is in fact a reasonable rate? How do they know that you haven't made this figure up? How do they know that you are not just 'trying it on'? Having it in print will make it appear credible, more believable and more legitimate.