are assigning action overrides to Lightning components it is benifical to only see those that support the object they are relevant to. Allowing them to select any component that implements the lightning:actionSuppor
t interface (regardless of object) can add confusion
and force developers to write extra code
to mitigate the wrong component being selected.
Unlike Visualforce page overrides that are explicitly linked with their object at development time and only useable with the stated object, your component can actually be assigned to actions on any object. The docs remind you of this fact and offer some best practices
The sfdc:object qualifie
r in the .design file is used by developers and Lightning App Builder to filter components to only show up in the design for relevant record pages. This same approach could be used to filter components shown when assigning Lightning Component Bundles.