What is the purpose of entity_keys in ContentEntityBase?
entity_keys are basically an alias for fields. It allows generic code, like the storage backends, to access common fields that have different names, for example the ID. They need write the saved value, so the can't just use id() as most places.

Internally, those keys are also used by generic methods like id(), label(), getRevisionId() and so on, so they work out of the box.

There isn't really a reason that comment doesn't define status. Nothing really uses status, if you search for getKey('status'), there are no matches, while getKey('id') finds a lot.

Node only uses it internally for the isPublished().