Apple - Why is the Finder app always open?

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

It's not only a "file browser", but it's responsible for quite a bit of GUI functionality such as the Desktop, following paths and connecting to servers. Finder is always running, and a launch service will relaunch it if it is quit unnaturally. Much like File Explorer (explorer.exe) on Windows, you shouldn't quit it. Quit functionality can be added back to Finder, but you really shouldn't unless you have a very good reason for it.

Think of the background process for Finder akin to that of the 'Windows Explorer' background process on Windows. As mentioned above, it is essential for the MacOS UI & GUI elements on the desktop. If you are new to Mac, take a look at the Activity Monitor App in your Utilities folder. This is essentially the Mac equivalent to Task Manager on Windows and can help organize/debug processes in the background.