It's possible that using high quality templates in your project to give it visual interest might make your project more "engaging" for your stakeholders, giving you a competitive advantage. Templates can raise the perceived professionalism of your project: The same way that a suit might make you appear more professional when going on a job interview, the use of project templates can make your project appear more professional.
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.