Laying down for the benefit of all sentient beings

I heard a Dhamma talk wherein a monk said that Mara blamed the Buddha being lazy cause he laid down and the Buddha replied something like "I am laying down for the benefit of all sentient beings". Can someone explain the logic behind his answer and expand on this issue? Is there a practice wherein you gain a mind set to take action for your own good while thinking you are doing every action? ie. "for the benefit of all sentient beings"