In reply to Jason Witt.

The goal is to move all of the "appearance" things to the Customizer, and to encourage plugins to put any options that change the front-end in the Customizer. The Customizer, simply put, offers a much better and more streamlined user experience than the admin, primarily because of the live previewing functionality.

The other advantage to the Customizer is that it's easier for themes and plugins to integrate their options there than in the admin, in a way to provides a seamless experience for users.

If you've ever looked into the menus code, you'll know that it is a complete disaster. Menus in particular need a complete rewrite anyway, which is why the Menu Customizer project is important.