Why does a turtle have a hard shell? Why do tigers have large teeth and claws? Why do some plants develop spikes? Why does a cuckoo chick push other eggs out of its parasite nest? Why are toxic animals often brightly coloured? Why are there non-toxic butterflies that have a similar appearance to toxic species? Why are mammals in the Arctic often white? Why do some insects resemble plant material? Why does a spider make a web? Why do birds make nests? Why do trees in rainforests grow high in the sky with only leaves growing on the top?
Any number of these questions could be asked, and they would be meaningful questions with real answers. The answers are all found in the context of evolution. A tiger's teeth need to be able to rip flesh, as to be a tiger means needing to eat flesh. If a tiger were herbivorous, then we'd expect the predatory features not to be there.
Yet here I am thinking about the problem in terms of rationales. My bias as a sapient creature is seeing things how something with a mind would. Of course it's obvious to me that a tiger's teeth are for ripping apart flesh, just as a herbivores are for grinding plant material, just as a spider's web is just like our fishing nets for trapping pray. But unlike our fishing net, what does a spider know about building a web? Does a spider know that it builds a web in order to catch prey? It doesn't know anything, it just does it. So the story, it seems, is an invention of our own minds.
Enter what Dan Dennett calls free-floating rationales. Of course a spider builds a web to catch its food, just as a tiger has evolved teeth that are better at ripping flesh. These rationales exist, but aren't represented anywhere. The rationale is a maximisation of function, spiders that have better web-building genes do better than spiders that don't. Thus over evolutionary time, we should see spiders that are great at building webs because they're the ones that pass on their genetic material.
Perhaps then, it is our mind that has been shaped by natural selection, rather than our mind projecting onto nature. The obviousness of the structure of the tooth as being for the purpose of tearing through flesh might be just as obvious as thoughts that a tooth is for tearing through flesh.