Relationship between Shapes graph and owl:Ontology

Hello

Is a Shape Graph always an owl:Ontology ? (no, from the spec)
Why is it "often" (to quote Holger's last email here) declared as an
owl:Ontology ?
What are the advantages of stating it is of type owl:Ontology, if the graph
does not declare any new classes or properties or OWL axioms, but only
Shapes constraints ? and isn't this declaration misleading ?
Is this declaration an agreed best practice ?

Thanks !
Thomas

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