I've just been looking through The Really Simple Guide to Humanism, (which is an excellent introduction btw), and in particular a video on the question Without a God, or religion, why should we care about anything? About 2/3 of the way through, Colin Blakemore brings up an old point that science should address questions of how and leave questions of why to religion. Of course, in the context of humanism, this means the why questions are suspicious back doors into religious thinking. But I fundamentally disagree. And here's why.
Why addresses issues of greater complexity than how typically does, but that does not mean the question presupposes that the answer makes reference to an intelligent agent. And the biological answer to why am I here is substantially different from that to how did I come to be here.
How did I come to be here prompts an answer which addresses the biological mechanisms around reproduction. It may include sexual dimorphism, meiosis, fertilisation, mitosis and embryology. It is a perfectly valid discussion to have, but has a narrower scope than discussions which why questions prompt.
Why am I here prompts a biological explanation which is far more profound, and addresses fundamental questions of existence, which are not the exclusive domain of religion. For example, I may answer that you are here because your parents succeeded in making you, that their parents succeeded in making them, and that every one of your ancestors, unlike most of their competitors, succeeded in leaving offspring who also succeeded against the competition to leave offspring. I could go on to point out that some of the reasons your ancestors succeeded was because they happened to have combinations of genes which made them well adapted to successfully reproduce in the environment they lived in. Finally, we could consider that your ancestors were also successful, in part, because they passed on the genes which gave them their success, and that you are likely to possess many of those successful genes yourself.
So why am I here can prompt a discussion of the fundamentals of evolution which are often beyond the scope of how questions. Why would any thinking person want to dismiss that?