The reason I think that window managers are necessary is some of the simple functions like focus management; Windows and OS X get around that by not supporting any useful policies, but it's highly unlikely that a set of applications would all agree on the behavior of my chosen focus policy. Furthermore, I have window manager hotkeys for things like window-shade, maximize, and iconify, and I've intentionally selected key combinations that applications don't generally detect (e.g., pressing both shift keys) so as not to interfere with application key usage.
For that matter, window management stuff can be important on a cell phone; while you're on a call, you should be able to put the call management application into a small portion of the screen and use arbitrary other applications in the majority of the screen, while still being able to control the call without interfering with the other application. For example, you're on a call, and you have to take notes, so you put it on speaker and go to a editor; while you're in the middle of taking notes, there's noise in the room you're in, so you need to mute, and later someone asks you about something in the notes, so you have to unmute, read from the application, and mute again. Phones I've seen don't provide a way to do this sort of stuff without needing to find the application (and sometimes document) again each time you do phone things. Outside of the desktop, there's little call for the ability to have a dozen ongoing tasks that you switch between, but there are situations in which you want two or three.