Why steemit has the 30 minute rule?

The 30 minute rule
At 0 seconds after the post is made, you are penalized 100% of your reward. ( This decreases linearly till 30 minutes after the post, where the penalty is 0. The penalty fee goes directly to the author. At this moment, the penalty is linear. I.e. 100% at 0 seconds, 50% penalty at 15 minutes, 0% penalty at 30 minutes.

why this penalty exists?? Can anybody explain, thanks.