Originally posted by Cenarious:
if he already got perfect enlightenment? i tot after a certain stage it is irreversible.
Yes, after the Eighth Bodhisattva Bhumi it is already irreversible, and Eighth Bhumi is also known as the non-retrogression stage, or 'Bu Dong Di'. And from the 10th Bhumi it is known as "The Path of Non-Meditation", which means no more training is necessary.
But why should Buddha meditate? I think firstly it is to set a good example for other Buddhists to meditate, it shows a balance between mundane life (of preaching the dharma, etc) and absorption in Nirvanic bliss, secondly meditation has a re-vitalising effect and the Buddha is thus able to rest his body, in fact I read somewhere that a short while of meditation for an advanced meditator can equal to hours of sleep. The Buddha is known to sleep only 1 hour a day. I think he meditates more than he sleeps.