In days past ships had names if for nothing else to distinguish themselves from other vessels. When Portland was a busy merchant port there was a section in the paper called the port calendar. It would list the ships that were to arrive, those in port, and those about to depart.
Port calendar 6/2/2015
Arrivals
Pacific Bear from Yokohama
Java Mail from Manila
M.M. Dant from Hong Kong
In port
Santa Ana Grain Dock
Oregon Mail T-2
Hawaiian Monarch T-4
Departure
Tama Maru to Nagoya
Alaskan Rambler to Seattle
Loyola Victory to Da Nang
There were a lot of merchant seamen in the Portland area. Family members would know which ships there relatives were on and they would keep an eye on the paper for their arrival. Mail was also received by the ships name. The mail would be sent to the shipping line's foreign port agent, addressed to the sailor by the name of the ship he was on. Upon arrival in a foreign port the agent would meet the ship with the mail and orders from the company.
Different shipping companies would name their ships with a certain theme. Pacific Far East lines Golden Bear, Pacific Bear, Java Bear, American Mail Lines Canada Mail, Oregon Mail, Japan Mail, Grace Prudential Lines Santa Ana, Santa Cristo, States Lines Oregon, Washington, Colorado. Matson lines Hawaiian Citizen, Hawaiian Rancher, Hawaiian Farmer and so on. If you knew the ships name you could just about tell which company owned the ship.
For other practical reasons on small boats GaryP1958 nailed it. Hailing other vessels and the Coast Guard in an emergency. That is why when naming a boat it's important to come up with a reasonable name that can roll of your tongue in an emergency. I named my boat Teresa D after my first mate.
Roger thanks for starting this thread it should be a fun one.
Picture SS Permanente Silver Bow owned by Kaiser Permanente Cement company