Really depends on how you use a PC. Example for multiple desktops, have one with certain apps open for personal use, and another with all work related things on it. Instead of trying to organise everything on a single taskbar trying to keep track what's personal and what's work, with multiple desktops you can switch between the two.
Of course it's not for everyone, I usually have minimal windows open anyway so it doesn't benefit me. But for those who have a lot of windows open, it can help organise things.