This might be obvious, but it still bears saying: templates are a natural for navigation. Navigation is the directory of your site, like buttons on an elevator, that take you through the main levels of your site.

Beyond the homepage, users want the security of knowing that the primary navigation will always be available throughout your site. Templates provide this consistency for your users.

And for the sites that require additional navigation; templates allow you to create 'nested templates' based on the main template that provide secondary navigation for given sections of your site.

"I think we talk about Web navigation because 'figuring out where you are' is a much more pervasive problem on the Web than in physical spaces."
–Steven Krug, "Don't Make Me Think: A Common Sense Approach to Web Usability"