Why Am I Here?

That question has been asked uncountable times by uncountable numbers of people, ever since humanity was self-aware enough to pose it.
Now I'm asking, like so many before me.

Why am I here?

Did I come here by choice, or is it a penance of some sort? Was I accidentally left here, and never retrieved? Is this the version of hell or heaven for an alien race?
Regardless of how I came here, one might ask concurrently, what am I to do here? Or to rephrase the question, what is my purpose while I am on this small chunk of rock, third out from an M-Type star of 1.9891 * 10 ^ 30 kg in mass? Or to not be so technical,
what is my purpose here?

I can break that down to two (very) general purposes: To have a use, or to not have a use. To be useful or to not be useful. How does one define being useful, or not? Is being useful to pursue things that better oneself, or to better others? Is not being useful something as simple as sitting around all day, not contributing anything to the betterment of the whole, to the human collective, for lack of a better term? Or by doing that, is one being useful because if they actually got up and did something, it would be detrimental to the human collective? As an example, what if Hitler had been a lazy good-for-nothing who never really did much? The world would be a different place today. Would it be a better place? I don't know.

Let's assume that I'm here to be useful. What it useful? Is it something I can do to better myself, but not others, or something that betters myself and others? If being useful is doing something that betters myself and others, how would I do it? Would doing something that is ethically or morally wrong, but still benefiting to a large number of people, be considered useful? As in, do the ends justify the means, or do the reprehensible means nullify the benefits of the end result?

And why do I get so philosophical at night?