What it's for
Port 515 is the default port for LPD (Line Printer Daemon), one of the earliest network printing protocols, originating on Unix systems. It allows a client to submit a print job to a remote printer or print server, and remains present today mostly for legacy compatibility — many modern printers and operating systems support LPD alongside newer protocols purely so older clients and software can still print.
Protocol basics
LPD runs over TCP, since reliably transferring a full print job (which can include arbitrary binary data like a PostScript or PCL file) benefits from ordered, guaranteed delivery. The protocol's command set is intentionally simple: submit a job, query queue status, and remove a job — a design reflecting its origins in an era of much simpler, single-purpose network printers. Most modern environments have moved to IPP (Internet Printing Protocol, port 631) as the more capable and actively developed standard, with LPD retained mainly for backward compatibility.