Some time ago, I read about Facebook being accessible over Tor and I couldn't ignore that their official URL https://facebookcorewwwi.onion/ not only shows HTTPS but a valid certificate issued for a .onion domain (actually the first in history).
Then the question came: given the nature of the Tor protocol, encrypting peer-to-peer communication, why using HTTPS? What advantages does it add to plain HTTP?
I'll share my conclusions in the answer
Facebook give reasons for this in their blog when annoucing the TOR address
We decided to use SSL atop this service due in part to architectural considerations - for example, we use the Tor daemon as a reverse proxy into a load balancer and Facebook traffic requires the protection of SSL over that link. As a result, we have provided an SSL certificate which cites our onion address; this mechanism removes the Tor Browser's “SSL Certificate Warning” for that onion address and increases confidence that this service really is run by Facebook. Issuing an SSL certificate for a Tor implementation is - in the Tor world - a novel solution to attribute ownership of an onion address; other solutions for attribution are ripe for consideration, but we believe that this one provides an appropriate starting point for such discussion.