I find those things full of purposes:@mountainheart
My answer to each of those is "there isn't one" or possibly "because that's what it does" if I'm feeling circular.
The difference between your examples and humans is our large degree of sapience, our capacity for reflection. Perhaps the search for meaning or purpose speaks of a desire for guidance in how to live, given that we are capable of considerations which a maggot is not. It simply is what it is and does what it does. We have infinite choices, some constructive to ourselves, others and our environment, and some destructive. So perhaps a/the purpose is to become our uttermost selves, the quintessential expression of human-ness. This, I believe, may echo the sentiments above about the purpose of life being to simply experience it fully.
The concept of "the quintessential expression of human-ness", of course, leads to a whole 'nother set of questions!
What is the purpose of a leaf bursting from a bud? My purpose in life: to change, to grow, to transform with the seasons, to unfurl to the fullness of what I can be and am capable of.
What is the purpose of a tree bending in the wind? A purpose in life: to learn resilience; to fully face the storms that come and to bend rather than break; becoming stronger in the process.
What is the purpose of a blackbird singing high in a tree? My purpose in life now: to sing a song to you.
What is the purpose of a dog lying in the sunshine? A purpose in life: to seize every moment and to enjoy what can be enjoyed; to fully experience what can be experienced; to just take time to be; to live in awareness.
What is the purpose of a maggot eating within a corpse? The purpose of life. To take part in the great cycle of life; to be born; to live; to die; to be transformed.
I find your statement "It simply is what it is and does what it does" pretty much sums up what life is all about for me Human's aren't special IMO: we are animals and when we remember that and get in touch with our inner animal I think we are on the way to understanding our purpose.