The answer to your question is that in asexual reproduction there is no exchange of genetic information to create offspring that are different from the parents. In the case of plants, the new plants created are clones of the parent plant. It is important for genetic information to be shared so that the new plants created have genes which are different from the parent plant. If the new genetic make-up is better than that of the parents, the plant is able to flourish better and the species evolves.
Now, self pollination is a process where the same plant provides the pollen and the egg for the new plant that is created. Though this is not as efficient a way of improving the genes of the plant as cross pollination, it is in a way better than asexual reproduction.
This is the reason, plants only self pollinate when they are unable to cross pollinate. This could be due to a small number of other plants of the same species in the surrounding environment or no medium to carry the pollen from one plant to another.