Water buffaloes are prized animals for their work in the fields. Given their value, they are sacrificed after an older person dies to ensure that the person's soul makes it to a better place.
The water buffalo is then eaten. Given their usually long tenure in the fields, the meat is often as about as tender as boot leather. We know this because we once ordered chicken curry, only to find after a few strenuous chews that unbeknown to us, buffalo was happily substituted.
Read more about the hill tribes and trekking in Sapa.