Because of biodiversity. A lot of different plant and animal species are there, many still undiscovered. One of those plants may harbour a cure for cancer or AIDS.
Because they act as a respiritary system for the planet, exchanging gases like carbon dioxide and oxygen. This is true not only of the forests, but the seas. Plankton swarms produce more oxygen that the rainforests.
And of course the forest holds the soil together, preventing it from eroding away.