@ntnsndr you can have reference material and the work you're working on side-by-side without sacrificing information density or having to rescale your window.
@Demosthenes This is as good a reason as any. @ntnsndr IMO you can multi-task effectively if you are actively focused on just one task while the other(s) are passive. It's more difficult to keep the passive task(s) going at the same time if you have to keep minimizing windows or switching workspaces. It's easier to just have two screens.