I have never heard of this before. What exactly is the point of it?
It's a wonderful expression of pessimism and expectations of impending doom.
Pretty much the closer it is to midnight the closer humanity is to killing itself off. Originally it was a cover for a story featuring the 'omg we're all gona die' nuke war of the 50s which never occured, then they made it it's own thing. I think it's been moved as far back as 14 minutes from midnight at one point, and that where it is now is about as far forward as it's ever gone.
As a note, despite this we're still not dead. It's just an exersize in symbolism taken to a really sad extreme.