I am hesitant to get back into that discussion as that question has been treated in a number of topics and for me the conclusion from those debates is that such a feature apparently makes sense for some communities but not for others. I don’t want to convince anybody that they need this, but I do think that there are a number of reasons what you or a community might want this.
Here are some examples (quickly selected, so I may have missed some important ones):
https://meta.discourse.org/t/impersonation-and-reading-private-messages/8485/23?u=tophee
https://meta.discourse.org/t/impersonation-and-reading-private-messages/8485/18?u=tophee
https://meta.discourse.org/t/restrict-moderators-from-reading-all-pms/8618/9?u=tophee
https://meta.discourse.org/t/impersonation-and-reading-private-messages/8485/9?u=tophee
My aim here, however, is to see if and how this can be implemented as a plugin.