Usually, mail to hosts with fully-qualified domain names is delivered via Internet style (SMTP) delivery using Domain Name Service (DNS), or via the relay host. The uucpxtable forces delivery via UUCP routing by converting the domainized name into a UUCP-style un-domainized remote hostname.
It is frequently used when you're a mail forwarder for a site or domain or when you wish to send mail via a direct and reliable UUCP link rather than potentially multiple hops through the default mailer and any intermediate systems and networks.
UUCP sites that talk to UUCP neighbors who use domainized mail headers would use this file to force delivery of the mail through the direct UUCP point-to-point link between the two systems rather than using the less direct route through the RELAY_MAILER and RELAY_HOST or through the DEFAULT_MAILER.
Internet sites who do not talk UUCP probably would not use the uucpxtable.
Suppose you provide mail forwarding service to a system called sesame.com in DNS and sesame in the UUCP maps. You would need the following uucpxtable entry to force mail for their host to go through your direct UUCP connection.