I was thinking of extinct animals. (Maybe I should have said "paleontologists".) Whenever there's a structure or physical feature in a fossil whose purpose we can't identify, the default assumption is "it was probably used for mating displays." Sometimes further study (or comparison to extant species) comes up with a more practical use later. It's all hypothetical, of course, but so is "religious purposes." :)

On this note, a vast amount of human energy (surplus and sometimes not, in the aggregate) can be understood as mating displays. Of course, we have added gender to the equation, which complicated things quite a bit.