Why do some people sit on blocks or pillows when they meditate?
Sitting on a block, pillow, or anything slightly elevated makes it easier to balance while sitting cross legged with a straight back, and makes it a little bit more comfortable.
I usually sit on a yoga block, which I find for me is the perfect amount of elevation. It allows me to sit comfortably with a straight back and with much less effort. This allows me to actually sit and meditate for longer.