Variability is essential to survival, adaptation, and evolution. Genetic recombination of DNA ensures continued variety which allows all species the best chance of survival in various environments. All species of plants and animals are ridden with variety to ensure survival. The variety allows for the process of natural selection to occur.
Natural selection, or survival of the fittest, is based on the idea that organisms with the best adaptations for an environment will survive and reproduce. The organisms that reproduce are able to pass on their genetic code. Suppose you transplant multiple flowers into a new environment. Some will survive better than others providing them the opportunity to pass on their genes and continuing to flourish.
Different flowers are adapted in different ways to survive in the environment they grow in, and according to the insects, climate, creatures and conditions they require for pollination. For example, a cactus sometimes has flowers. The cactus has adapted to be a reservoir of rarely obtainable water, storing it in its fleshy leaves and protecting it from preadtors through dangerous often toxic spines. This protects the flower, so that insects/creatures can visit it long enouth for pollination to occur. Som,e flowers have adapted by colour - the insects in their particular environment visited one shade more often than other shades and so the strongest colour won out down the generations of seed. Other flowers use scent.
A rather impossible question to answer. It is like asking why the nature is the way it is. This nature is full of variety in its every aspect. Like there are not only so many different kinds of flowers, but there are so many different kinds of so many things like leaves, roots, fruits, animals, fishes, birds, insects, and so on. If we look at just the human body we find such wide variety of organs, cells, chemicals and processes in it. I believe there can be no single correct answer to the given question. Variability is kind of a rule of nature , and the variety of flowers is just an instance of the nature's rule of variability.
Also it is not possible to compare one flower with others in terms of being better or worse. This is because, we have no objective criteria of what makes a flower good or bad. Flower of each plant is best suited to serve the needs of that kind of plant. Even if we thing of flowers as good or bad in terms of beauty and fragrance, it is not possible to make objective comparisons, because these things are very much a matter of individual subjective preferences.