From the back of the book:
The Bestiary is the first "Hero System" supplement designed for use with any game in the Hero System. Animal encounters occur in all branches of action literature, so they're appropriate for any of Hero Games' RPGs.
Animals add to the background detail; creatures can contribute inestimably to the flavor of a scenario. The lost valley that the Justice, Inc. explorers have slogged through the jungle for weeks to discover would hardly be worth the trip without a few prehistoric monsters to marvel at. A Fantasy Hero campaign without dragons or unicorns is almost unthinkable. That winged snake on the other side of the interdimensional portal is a quick and easy way to inform the Champions superhero that he or she has taken a wrong turn...