Templates save you money: If you use a royalty_free template, it likely costs less than hiring someone to create a custom graphics package for you. You could easily spend hundreds (or possibly thousands) of pounds on a custom motion graphics opening, for example, whereas you can frequently purchase a royalty_free template for under £200. Of course, the royalty_free template isn't as unique as a totally custom template, but it definitely costs less, and you could always customize the royalty_free template to make it more unique for you.
When new freelancers are creating a business proposal template that they are going to use often, they usually miss the most important pieces that make the difference between a good proposal and a proposal that sells. By emulating the sales process, you can create a proposal that solidifies the relationship that you've built with a prospect, and reinforces the close of your sale.