Nuts, and something beautiful to look at!
Beside the already mentioned oxygen, they supply wood for housing. And shade to help keep us cool. Some provide medicine, such as the willow with salicylate used for aspirin, or the camphor tree. And that all important morning beverage, coffee!
Oxygen, carbon dioxide and a means to temporize the environment by offering shade and shelter from winds, and to also take impurities out of the air.
Shade, the joy of climbing them and building a play house in them. Trees love humans and humans love trees.
Trees provide oxygen, save water and also trees provide food.
Shelter, shade, oxygen, homes for animals, beauty.