Why sitting meditation, and is vippasana and breath meditation sufficient?

**Why** do we have to watch body sensations separately by **sitting meditation**, when even in our daily life we are experiencing sensations moment to moment (and realizing everything is vibrating in our body)? And can we achieve omniscience and knowledge of our past lives, karma, etc., by watching breath and watching body sensations only? Or there is more to it in vippasana meditations, other than watching breath and body sensations? --- (This question was originally titled "tibetan buddhism chanting om mani padme hum and watching the breath meditations and watching mental states")