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Speaking from my own experiences, and pulling loosely from the zen tradition in which I practice:

Sitting meditation keeps you more alert, makes it easier to reach a state of samadhi, or focused, non-discriminating awareness. For a while, due to back pain, I switched to lying meditation. I was not a fan, it was much easier to drift off. Zen meditation typically involves leaving the eyes partly open to avoid daydreaming or entering some sort of trance state. I also found this difficult to maintain while in a prone position.

Sitting in full lotus is difficult for me, and I'm relatively flexible. I generally sit half-lotus or in a chair. Sitting without a chair for long periods causes pain in the legs and back/shoulders. That's part of the practice, learning to maintain focus and equanimity through pain and discomfort.

To the best of my knowledge, the common sitting postures - lotus, half-lotus, burmese, seiza - came about because they were maintainable for long periods. Sitting indian style with legs crossed in front, for example, your stomach muscles will quickly tire and you'll be hunched over.