The Alternate Sidebar Page template will show your page with a sidebar that is on the opposite side of your default sidebar location.
When you set up your Business Identity site inside of the Customizer, you are given the option of having your primary sidebar show on either the right or left side of your site. Depending on which option you select, the Alternate Sidebar Page template will show the sidebar on the opposite side of what you’ve chosen in the Customizer.
Here’s an example: inside of the Customizer you’ve chosen Right as the default sidebar location, which shows all of your primary sidebar content on the right side of your screen.
By using the Alternate Sidebar Page template, everything on your page is the same except the location of the sidebar, which is now flipped to the left side of your screen.