I can’t find detailed discussion about this now, but IIRC, the main rationale for this is to leverage the community for front line moderation support. In particular, the team was concerned about two types of destructive behaviors, mentioned in the linked posts:
https://meta.discourse.org/t/improvements-to-self-deleted-posts/8564?u=sailsman63
The first, in particular, can be hard for staff to track in a busy site with many users. Allowing normal users to see withdrawn posts can get extra eyes on potential trolls and bullies, and a few flags on “NOT OK” content can alert staff much more quickly than if they try to review all withdrawn posts and other activity.