When I switch to the Finder using cmd-tab, no open Finder windows show up.
I switched to finder with cmd-tab, Finder is brought to the front, but its windows are not.
The Finder stays "open" even if windows are not open/present, and when switching to the Finder icon via cmd+tab and no windows show, press cmd+ n and a finder window will open/appear.
So, if you have no finder windows open, when switching to the finder app using cmd+tab, it will do nothing, besides being active (check menu bar and you will see Finder menu options), which allows you to press cmd+n to open a new window. Once windows are open, using cmd+tab will switch to any open window. Same goes for most applications as well, ie: Firefox, Safari, etc..