Why

Why not?

Serving a biological purpose means, in effect, serving an intelligent purpose, if, as I believe, all biological functions were self engineered intelligently in anticipation of attending properly to expected problems.

And chances seem quite good that all systems functioning in the universe were self-engineered, even though more by reactive than pro-active choice, to nevertheless have, and thus serve, an intelligent purpose.
Which may be why so many are confused as to the use of the word purpose, since service of a purpose still implies a purpose somewhere behind the service. And we're taught that, outside of man and beasts at least, the universe has no purpose for its actions. But it seems to me our inferential feelings are correct after all, and that service of a purpose is always for an intelligently acquired and required reason.