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.
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.