Why is sand or soil used in purificationof water

Sometimes sand or soil is used in purifying water where some quantity of soil or sand is put in a container then dirty water is poured into it. After a while the water goes down clean.

This is due to mechanical and biological action in the sand layer, slow gravity sand filters remove bacteria as well as small particles from water, making it safe to drink. This is a basic introduction to the subject and should help maintain a constant supply of clean, running water using simple technology. The sand filter described,is designed for domestic use only.
How the filter works--
The water passes through the sand from top to bottom. Larger suspended particles will settle in the top layers of sand. Smaller particles of organic sediment left in the sand filter are eaten by microscopic organisms including bacteria and protozoans which 'stick' in the layers of slime that form around the sand particles. The clean water which passes through the filter is safe to drink. If the grain size is around 0.1mm in diameter, a sand filter can remove all fecal coliforms (bacteria that originate from feces) and virtually all viruses.