Why do peacocks grow such large, conspicuous tails? Why do people take degrees in subjects like Philosophy or Classics, despite these subjects having no obvious practical value? Why do people take pains to avoid splitting infinitives, even though everyone can understand split infinitives perfectly well?

The second example seems to fit in worse than the two others, given that plenty of people take degrees in such subjects due to a love of learning and due to being genuinely interested in them.