The next box on the Navigation Properties dialog isn’t quite worded correctly. It says, When I click the upper-left corner, switch between my recent apps.
If you check that box, yes, clicking in the upper-left corner switches to the Metro app that you most recently had open, but it does more: Checking the box enables the Metro Task Switcher, which intrudes on the left, as you can see. Unchecking the box turns off the Metro Task Switcher entirely.
I never use the Metro Task Switcher, so I uncheck the box. If I want to switch between running Metro apps or desktop windows, I use Alt+Tab, the CoolSwitch that’s worked since Windows 98.