[gizmo686]

It would give you a certificate chain which may authenticate the onion service as being operated as who it purports to. Of course, depending on context, a certificate that is useful for that purpose might itself be too much if an information leak

[huhhuh]

DV certificates (that lets encrypt) provides offer no verification of the owner. EV certificates for .onion could be actually useful though, but one generally has to pay for EV cert.

[andrewaylett]

A certificate that's valid for both a regular domain and an onion domain gives you a degree of confidence of common ownership.