While your hero and heroine are naturally the most important creations you'll devise, they aren't likely to be the only flora and fauna on your new planet.
Animals in particular serve several purposes. Many cultures have pets. Almost all humans consume some form of animal, whether it's milk, cheese, meat, or even all the varieties of fish and seafood. And of course vegetation is good for shelter, food, clothing, possibly even transportation.
But what type of flora and fauna will you create? Part of your decision goes back to the general environment. If it's a desert planet, then there will be less flora and your fauna must necessarily be adapted to the desert environment. On the other hand, if it's a rain forest, you'll likely have a much greater variety of possibilities.
Animal creation can be a fascinating process. Imagine--you even have the honor of naming your creation. In the Mystic Valley books, I have an entire cast of animals ranging from the grimahrs, hoppers, peekies, and rowans to wolvalas. Grimahrs are very large wild catlike carnivores that have various combinations of blue-green, blue-purple or green-purple stripes. Rowans are horse-cow type of animals used for meat, milk, dairy. They produce great quantities of manure used by farmers. That last not so interesting fact actually is important in the valley as that manure is a barter item between the Rowan farmers and the grain farmers.
The flora in Mystic Valley ranges from quotania bushes (useful for everything from scent to jelly) to the wachaz thickets (useful for medicinal teas). There are mahlzal trees with black iron hard wood used in the giant looms and cucazhas that resemble cucumber or zucchini squash. Variety is everything...
Some people ask me how I keep track. That's simple. I keep a running glossary. That glossary begins on day one and is an ever evolving list that is open to additions all the way to the end of the book--or even through the next two or three books if I end up writing a series.
What about you? How do you envision your alien planet? Is it green with thousands of wonderful plants and animals? Or is it a desert with some really strange adaptive flora and fauna?