Multiple workspaces? You have me there. Then again, I have never heard most people wish for that.
Those people probably have never used a multiple workspace -capable window manager. The benefits only show when you've adjusted to having a large number of open windows being feasible. Once you do keep enough windows open that it's not sensible to keep them in a single workspace, you can organize them by purpose. For example, on my work computer my default configuration is email and browser in one workspace, IDE and other dev tools in another, testing environment in a third and a fourth workspace that gets used for whatever tasks I don't want cluttering up the other workspaces but aren't big enough things to create a new dedicated workspace for.
Now, I could make do without multiple workspaces, but then my single workspace would be somewhat cluttered and I'd have to close programs I'm not constantly using to keep it from getting really cluttered. And that adds overhead as I then have to reopen the programs and either wait for them to restore their state - if they even can - or manually set them back up.