There are all sorts of reasons for wanting to show certain menus to some users but not others.
As many visitors to WPMU DEV often build or run sites for clients, let’s take a simple client example.
Let’s say you have a client that needs things to be as easy as possible. This client accesses the site by going to the homepage first. They can easily get to the admin area from there, but what if they aren’t logged in? On many sites, there is no easy link to the login page.