In a theme with no menu locations defined – or a WordPress install with no menus set up – the default fallback option for menus is to display the output of wp_page_menu().
By default, this will output all of your pages firstly by their order, then by their title if the orders match.
This makes it simple to avoid the extra step entirely of configuring a menu, which is great for small sites – while still having the ability to get things in the right order.
Even on a site with menus defined, it can be nice to have the pages in the order one would expect to find them in, to make management easier. eg. I usually give the home page an order of -100 so it always appears at the top, and a Contact Page an order of 900 so it usually appears at the bottom.