Ah yes, I don't want anyone to eavesdrop on my pr0n!
Edit: to clarify, I find it weird that this has been limited to Private Browsing, when most people don't even use it for anything but adult websites. The feature is good and should be available for regular browsing too.
It can be enabled for all windows, but it was not enabled at all when the feature was first rolled out in v83. Presumably they wanted to allow time for any bugs to be shaken out before imposing it by default in any circumstance. It seems plausible they could enable it in regular windows as well in the future.
That way they can roll it out to all Firefox users, and if something breaks, users can always fall back to non-private mode until it's fixed.