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Location hiding

An onion service's IP address is protected. Onion services are an overlay network on top of TCP/IP, so in some sense IP addresses are not even meaningful to onion services: they are not even used in the protocol.

End-to-end authentication

When a user visits a particular onion, they know that the content they are seeing can only come from that particular onion. No impersonation is possible, which is generally not the case. Usually, reaching a website does not mean that a man-in-the-middle did not reroute to some other location (e.g. DNS attacks).

End-to-end encryption

Onion service traffic is encrypted from the client to the onion host. This is like getting strong SSL/HTTPS for free.

From here: https://community.torproject.org/onion-services/overview/

And there's another good reason for the Tor network: if you run an onion service, the traffic will use only Tor non-exit nodes in the circuit, giving a relief to the exit nodes.