[DorothySim]

Note that it is about TLS client certificates so it's not as widespread as it seems (unless you use these certs of course :) ).

[kroeckx]

It's not about client certificates, but it can be a reason why some software might want to do this. The return value of 1 allows the program to deal with verification errors instead of aborting the connection. You can for instance use this to display a proper error message that authentication failed, instead of just closing the connection and getting a generic TLS error message.

[adekok]

EAP-TLS. Wifi authentication for many enterprises and universities.

Most of which won't be using LibreSSL, tho...

[cesarb]

Not to mention TLS VPNs like openvpn.