In much of the Amazon rainforest destruction is in order to increase the amount of land that can be farmed.
Great areas are being deforested in order to grow crops.
The problem is, that the rainfall is largely caused by the rainforest. Therefore, by cutting down the rainforest you remove this rainfall.
Not only is rainforest destruction a dangerous thing for the community, but the land will be worthless within a short time of it being deforested.