What is the purpose of buttress roots?

Believe it or not but the soil in the jungle environment is actually very thin. The trees there cannot send down supportive tap-roots like oaks and other trees can, but have to support themselves with things like buttress roots or they will fall down. The roots also spread out farther than normal to get more nutrients from the soil. People who live in the jungle also use buttress roots for communication, by banging on them with long wooden sticks.