What it's for
Port 1723 is the port PPTP (Point-to-Point Tunneling Protocol) uses — one of the earliest widely deployed VPN protocols, notable mainly today as a cautionary legacy example. It was built into Windows early on and was popular for its simplicity and broad OS support, back when VPN adoption was much more limited.
Protocol basics
PPTP's control channel runs over TCP on port 1723, but the actual encrypted tunnel data uses GRE (Generic Routing Encapsulation, a separate IP protocol, not TCP or UDP), which historically caused headaches with NAT traversal since many routers didn't handle GRE cleanly by default. More importantly, PPTP's encryption (based on MS-CHAP authentication) has known, practical cryptographic weaknesses that have been publicly demonstrated for years — it should not be used for anything requiring real security today, with OpenVPN, WireGuard, or IPsec/IKEv2 as the appropriate modern replacements.