Why do telegrams print out "STOP" to end a sentence?

Never understood that. Reading a book, The Wednesday Wars by Gary D. Schmidt (it’s a young adult book but I don’t care), which is set in the 1967–68 school year in Long Island, and a teacher gets a telegram where STOP is printed out instead of using periods. That makes it very confusing, is there a reason why this is done that way?

BTW I’m almost done, and would give the book 4 out of 5 stars. It’s almost as good as the 5 of 5 perfectly written Pink and Green books.