In Stream synced as Object Type, how does CDC changes the behavior?

Looking at this page of the docs: https://www.palantir.com/docs/foundry/data-integration/change-data-capture/

It seems that one can setup a sync in “CDC” mode, so that the archive view of the stream will be deduplicated and only show the “latest data for each key”, which behaves a bit like a deduplicating View.

However, isn’t that the behavior of the Ontology even without setting up CDC in the stream itself ?
If I plug a stream to back an Object Type, if new primary keys are received, updates will be seen to the existing objects, isn’t it ? How is the CDC setup on the stream changing this behavior ?

Also, does the CDC setting has any cost implication ?

Thanks,