Do you need to add wp_nav function?

Because you can't change the html and css from the admin area. On a live site, if you wanted to make changes, you'd have to pull down a copy of the file from the server, edit it in your text editor and then reload it back to the server. Most people using Wordpress have no interest in making changes this way. They're much more comfortable using a graphical interface like the admin area to make changes on their site.

When you create or make changes to a menu in the admin area Wordpress generates the code for the menu. But how does it know where to put that code? What it does is look for a function called wp_nav_menu. When it finds it, that's where it places the code. You'll find this done repeatedly in Wordpress (and with dynamically-generated websites in general). Each page is made up of many smaller chunks of code combined together to make one page. There are many different hooks like the wp_nav_menu function that tell Wordpress what goes where.

Hope this helps. Good luck!