This is a (now obsolete
) journalistic convention
, to mark the end of a story
. At one time, journalists
had to submit their stories by telegram
; they would do so and mark the end of the story by "-30-
" - after which they could include personal messages
to their editors, such as (humor) "I want more whiskey
" or "I need money
Examples of these telegram-type conventions included "-20-" - more to follow, if you buy me female companionship, and "-10-" - a rumour (which often just meant, "need more money for the female companion in order to embarrass the local judge").
The previous paragraph is a lie. Actually, the "-**-" codes existed from "-10-" to "-90-", but only -30- is remembered nowadays. They existed as codes for journalists to tell editors what had to be done with the stories they filed.
It's truly a lost art.