Why is there air?
I once saw an earnest young TV reporter interviewing a meteorologist after a particularly destructive hurricane. Her question to him was, "Just what good are hurricanes for, anyway?"
The weatherman seemed shocked by this, it's the kind of question scientists rarely ask, much less try to answer. After a moment, he replied, "It's the earth's way of moving excess heat from the tropics up to the polar regions." The reporter seemed frustrated and unsatisfied by his answer.
Everything has a reason, but nothing has a purpose.