There are several reasons for doing a mixed species tropical tree plantation. From a technical point of view the reason is to produce more robust, stronger and faster growing trees. The idea is that if one only has one species of tree in a plantation, then those trees are reluctant to compete with their brothers and sisters for space, resulting in slower growth. However, if the roots around them belong to different species, then tropical trees will maximize their efforts to be the biggest and tallest in the competition for space in the rainforest canopy. This is of course desirable from a wood investment point of view.
Mixed species planting also means that when it is time to harvest, selective logging makes more sense. Since different species grow at different rates, only some trees are ready to harvest, meaning that clear cutting is neither responsible nor sensible. This way, as trees are selectively cut, others are planted within an existing forest, making for a healthier forest and a healthier environment. In my opinion clear cutting is a crime, because forests and woodlots have their own social networks. Many seedlings prefer to grow within existing forest, and struggle when planted on their own or out in the open.
A reason for mixed species cultivation is wildlife habitat. Amazonia Reforestation plants fruit trees and native species with plantation species. This permits endangered wildlife to make use of the plantation as additional habitat. Because of the native species and the fruit trees, wildlife can find food in this expanded habitat. One of the things I hate about teak plantations in Central America is that when one enters them they are dead. There are no insects, no birds and no animals. The reason is simple. Teak is a southeast Asian tree that is not native to the Americas. Since most insects and wildlife are niche dependent, mono-species teak cultivation offers nothing in return to its adopted home. The same goes for oil palms, which are a disaster for the environment and for endangered flora and fauna.