Why is the Finder app always open? First time Mac user. I am confused by the fact that the finder application is always open. For example, when switching apps with Cmd+Tab, it is always there, even when there are no windows opened for the app. Cmd-tabbing to Finder with no windows/tabs opened for it has no effect (it seems to me it would more intuitive to open a new tab when this action is chosen). 

Furthermore, there is no 'Close app' option when I right click the Finder icon in the dock. Am I misunderstanding something about the Finder app? Is it required for proper functioning of Mac OS? From what I see, Finder is just a file browser, which by no means should be forced to be open 100% of the time.
The main reason it's always open is that it displays the icons on the desktop. You can check what the finder does by enabling the "Quit" menu feature. To do this, launch the Terminal application and enter the following commands:

defaults write com.apple.finder QuitMenuItem -bool YES


Hit return. Then restart the Finder by running 

killall Finder


Close the Terminal. Click on the Desktop, choose "Finder" in the top bar, and "Quit Finder". Now you're running without Finder. First thing you'll notice is that all Desktop icons will be gone.

To get your desktop icons back, just click on Finder in the Dock.

If you want to remove the "Quit Finder" menu item, you can do that with:

defaults write com.apple.finder QuitMenuItem -bool NO