It's a `rollup' manager for Motif. If you don't know what a rollup is, try running rizzle, and then right-clicking on a window's title bar; here's what a rolled-up xterm looks like :
Maybe you're stuck with mwm because it is company policy. Maybe you actually like mwm (I do !) because it's a good window manager, it's got good industrial widgets, and it does what it says on the tin - it manages windows; it doesn't try and manage the internet, or even your computer, for you.
If, like me, you also like Motif's self raising windows and chunky padded corners, but often lose your dialog boxes underneath other windows, you'll appreciate being able to rizzle a window up into its title bar to create some space on a crowded desktop.
Another annoying consequence of `focus follows mouse' is that your keystrokes won't go anywhere if the mouse is not over a window (and when you've got a desk full of papers, the mouse will inevitably stray). This is where sloppy focus comes in useful (i.e. it's fine for the mouse to stray onto the background, the focus will stay with your window unless you enter another window). The only problem is, mwm doesn't do sloppy focus; well, not unless you use rizzle, that is.
Now that the Motif source code is freely available, there's nothing to stop you hacking mwm to make it do desktops and rollups like XFce and KDE do, but is that such a good idea ? Not really, the last thing you want to do is break Motif (even the bugs are too established to be fixed); and besides, you're only allowed to hack it on `Open Source' systems (i.e. Linux and BSD), not on Solaris, HP/UX, AIX, Digital Unix, or OpenVMS.
This is where rizzle is quite different - it's not part of the window manager, it's just another X application, so there's no need to do anything to mwm, it should work just like that. With a bit of luck, it might even work on VMS...