Comment by gizmo686
1 day ago
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
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.
A certificate that's valid for both a regular domain and an onion domain gives you a degree of confidence of common ownership.