Why do giant trees have 'buttress roots'?

The layer of rotting green leaves on the forest floor, called the humus layer, in the rainforest is very thin. In the hot, moist climate of the rainforest, the dead leaves or plants are decomposed to nutrients, which are absorbed immediately by the plants. However, in thin soils, the trees cannot anchor themselves strongly to the ground for a good support. That is why the giant trees resort to other ways such as buttress roots. These roots flare out like planks from the base of large trees. They provide support and cover a wider area for collecting nutrients.