Why do we use that -30- sign?

I promise more posts to come soon, I had a crazy week at work between deadlines and moving offices (yikes!) but I think this is the funniest NYT correction EVER:

An article on Thursday about the arraignment of three men in the shooting of two New York police officers, one of whom died, misstated the schedule set by a judge for a trial in the case. The trial is expected to begin by February, not by “Feb. 30.” The error occurred when an editor saw the symbol “— 30 —” typed at the bottom of the reporter’s article and combined it with the last word, “February.” It is actually a notation that journalists have used through the years to denote the end of an article. Although many no longer use it or even know what it means, some journalists continue to debate its origin. A popular theory is that it was a sign-off code developed by telegraph operators. Another tale is that reporters began signing their articles with “30” to demand a living wage of $30 per week. Most dictionaries still include the symbol in the definition of thirty, noting that it means “conclusion” or “end of a news story.”

My first writing professor told me to use it, and actually took points off if it wasn't on the page. He was major old-school, and I should have known my habit was a little behind the times when no one in my masters program seemed to know what I was writing there. But seriously, you'd think of all the papers to KNOW about the -30- sign off, the NEW YORK TIMES would.