4 Answers

Have you done any research?
Why the Buddha sits in meditation
There are several reasons to sit that way while meditating. The first is simply practical: if you sit for a long period of time, you could fall asleep or tip over. Sitting up in the lotus position is very stable and comfortable, once you get used to it. Another reason for sitting that way is that the position keeps the natural curves of your spine balanced, which means you won’t get tired or sore even if you sit for a long period of time. Not only that, but its easy to keep warm while sitting that way. There are said to be seven centers of spiritual energy or chakras in the body, and sitting in the lotus position facilitates the balance and flow of energy through those chakras.

See the above in Daren's comment.