A global page template that can be used across your site with different data.
For example, the layout for a Product Details Page would be created as a Master page and no matter the data brought into that page from your APIs, the layout of the page will always remain the same.
You can also create Master pages for those pages that are linked to from all across your site, for example, Search, Login, and Checkout Pages.
"I used created a template for all my Product Details Pages as a Master Page so they all have the same layout across my website."
For more info, see the Creating Master pages article.