The main reasons are for the timber and to clear land for farming.
"Most rainforests are cleared by chainsaws, bulldozers and fires for its timber value and then are followed by farming and ranching operations, even by world giants like Mitsubishi Corporation, Georgia Pacific, Texaco and Unocal."
"Experts estimate that we are losing 137 plant, animal and insect species every single day due to rainforest deforestation. That equates to 50,000 species a year. As the rainforest species disappear, so do many possible cures for life-threatening diseases. Currently, 121 prescription drugs sold worldwide come from plant-derived sources. While 25% of Western pharmaceuticals are derived from rainforest ingredients, less that 1% of these tropical trees and plants have been tested by scientists."
"On a small scale, slash and burn agriculture (by individual farmers) has no lasting effect on the rain-forest- it is a sustainable use of the forest. However, on a large commercial scale, where bulldozers and flame-throwers are use to convert huge tracts of land into pasture for cattle, the ability of the forest to recover is seriously impaired, destroying the forest for a very long time, if not forever. This kind of unsustainable destructive use affects the earth by taking a large portion of the biosphere out of production - which interrupts the carbon and oxygen cycles, affects local water and soil quality, and causes
a decline in the global gene pool by making many species extinct.