One frequent question I got from new users is "how to display more than one document side-by-side". The answer is of course sub-window (A.K.A tile mode). It is often used as a simple trick to view reference image, or copying elements between documents.

Krita's Multiple Document Mode option is hidden too deep. It's very difficult for people to find it without knowing it before hand. In real world, artists need to switch between Tabbed/Sub-window quite often, but it's very clumsy to do so in Krita.

Other programs like Photoshop integrates this option into Window menu (Consolidate/Tile). I wish Krita will have the same as well. We can even put it in the Workspace chooser button menu, that'll make it even easier to discover.