In MLB, if a Catcher drops or misses the third pitched strike, the batter becomes a runner and can attempt to reach first base before tagged out or forced out at first.
6.09 The batter becomes a runner when -- (a) He hits a fair ball; (b) The third strike called by the umpire is not caught, providing (1) first base is unoccupied, or (2) first base is occupied with two out; -- Official MLB Rules
What is the reasoning behind this rule? Why does it only happen if first base is unoccupied, and what's with that crazy extra exception of two outs?