We can assume that organisms were originally created to be best adapted to fulfil their place in existence, which does not mean that they were created to be reproducing machines or fitness machines. As for why the particular organisms were created that were created, the principle of unity and variety, theme and variations, seems to be present. Many body plans are illustrated and many ecological niches are filled. We have soft animals, animals with shells, animals with bones on the inside, moving animals, stationary animals, air animals, animals in the trees, animals on land, animals in the soil, animals in the sea, animals of various sizes, and so on. It is even possible that the Lord did not create all the animals from scratch, but may have taken parts of one animal to create the next one.
As for why the Lord used the same design for different purposes at times, and different designs for the same purpose, we can assume that the same design is sometimes suited for more than one purpose, and that for different organisms, different designs may be better adapted for a given purpose. We can also recall that there has been limited evolution and degeneration since the creation, influencing the life forms existing at present.
The Lord even created the marvelous hereditary machinery found in organisms today, by which life can adapt to changing conditions. It is a marvel of Divine wisdom that the entrance of death, not originally part of God's plan, could be used by Him in this way to enable life to adapt to the conditions of a fallen world.
For some predictions based on creationism, see Creatioist Predictions. For evidence that they have been fulfilled, see How Old is Humanity.