1. We are part of a complex computation. Anyone into speculative fiction will have come across this theory. The Hitchhikers Guide to the Galaxy will give you a good example of what I mean. In that book, the Earth... and all of us are a computer which has been assigned to find the answer to Life, the Universe and Everything. H2G2 may be a humourous take on this but underneath that humour, there are some very concrete answers to your question.
2. We are travelers. This is an answer that comes from a serious problem in space travel. The problem is that all living things have a limited life span. If one were to attempt space travel lasting several thousands of years, our range is limited by the life span of the traveller. So, we populate the ship with enough life forms to survive several generations. Even if the original travellers die, the descendants still have a good chance of making it to the intended destination. Read this wiki article for a clearer explanation: http://en.wikipedia.org/wi