What is the reason the source_topic_id that is added to internal oneboxes?

A link to OP on CURRENT topic

https://meta.discourse.org/t/what-is-the-reason-the-source-topic-id-that-is-added-to-internal-oneboxes/71722

A link to OP on a different topic

https://meta.discourse.org/t/inconsistent-onebox-rendering/70788

https://meta.discourse.org/t/inconsistent-onebox-rendering/70788