From the SSL_CTX_new man page:

"SSLv23_method(void), SSLv23_server_method(void), SSLv23_client_method(void)

 A TLS/SSL connection established with these methods will understand the SSLv2,
 SSLv3, and TLSv1 protocol. A client will send out SSLv2 client hello messages
 and will indicate that it also understands SSLv3 and TLSv1. A server will
 understand SSLv2, SSLv3, and TLSv1 client hello messages. This is the best
 choice when compatibility is a concern."

This will maintain backwards compatibility for those us that don't use
TLS connections ...