While in Belize I saw numerous species of trees with buttress roots. These roots are common in species of trees found in the rain forest, because the trees have fairly shallow roots. The soils in the rain forest are nutrient poor, so having shallow, but widespread roots is a strategy that allows the tree to acquire nutrients from a wide area.
However, shallow roots don't provide much support to the tree, hence the buttresses. Some of these specialized roots can reach 15 feet. These were well over the top of my head.
The buttresses also provide surfaces for other flowering plants, mosses, lichens, and ferns to grow.