I don't know if there's any benefit for the end user, but I think it helps with the reliability of the Tor network because onion sites are accessed by normal Tor nodes, not exit nodes[1]. This means that if you access the onion version of a site you're "relieving" the exit nodes from the extra traffic, which is good because exit nodes already have to handle a massive amount of traffic from all the requests to the surface web.

[1] https://community.torproject.org/onion-services/overview/

[1.5] http://xmrhfasfg5suueegrnc4gsgyi2tyclcy5oz7f5drnrodmdtob6t2i...