I think for many of us it is curiosity, to try to gain answers to the unanswerable questions, which in doing so helps propel the human race into the future. Mars is the first hurdle when considering space exploration.
Is it possible for there to be other origins of life?
What materials can be found on Mars?
Can Mars sustain Earth based life, and there is there prospect of colonisation?
We can only answer these questions if we investigate them!