Excluding animals who have been domesticated to provide for humans, and thus altered through centuries and millenia elective breeding, animals and also flora have a purpose. They just lack the self awareness to know what it is and pursue it purposefully.

Humans, on the other hand have a purpose which for the most part we eschew in order to abstract more addictive and unnatural "purposes" that make us miserable in the end.

In other words, it's human hubris that makes us think we need to have purpose and animals don't because we are so far outside our natural purpose via civilization that we think k everything we do to uphold civilization is our purpose when really it's more like we, as individuals, are acting more like once automous organisms now being pressured and reduced down to organelles of a larger organism (civilization). Very few of us can live without it or act against or even escape it.

Our purpose was to keep balance. We abdicated for something else that has removed us from ourselves a great deal.

By the way, we're not the only species to have remade the earth.