>Yes, I use pcmanfm on Linux and the spacebar will open the file in the default program.

That's not what QuickLook does. It allows the user to get a "quick look" at a file without launching a default application. Also, in macOS you get access to QuickLook from inside any application's Open dialog. That's a huge time saver when you have similar files and just need to see which one before doing a full open. Think large image files that you want to place in a layout.