A WordPress site becomes pretty useless unless you are able to navigate from one page to another. I guess if you just wanted to blog, with no other static pages, you could get along without a navigation menu. Some themes create the navigation structure as you create pages, while other themes require you to create at least one navigation menu.
You can include one or more pages in your navigation menu, but you are not required to include all pages in your menu. And, you do not have to maintain the page creation order. You can reorder the navigation menu as you like.
Below is the menu structure for the mainmenu.