There is an explanation in the Wikipedia article about the clock (written yesterday). The reason is to give some extra time to close the doors, and to also give the clocks a bit of more room to synchronize with the central clock.
Not really. The second hand takes 58.5 second to move the entire circle at constant speed if I recall correctly. Then it stops for 1.5 seconds before it starts moving again. During that 1.5 second pause you close the doors of the trains. So it is more a method of synching the closing of the doors with the clock rather than actually gaining any time.