“The purpose of life is to live it, to taste experience to the utmost, to reach out eagerly and without fear for newer and richer experience.”

Would you take a one-way trip to Mars? Think about that for a moment: would you leave your family, friends, the entire planet Earth behind to live out your days forever enclosed in a sealed habitat on a distant planet, entirely dependent on your fellow colonists and resupply ships from Earth? Here are some people who say they will:
It’s a thought-provoking short film which gives insight into the type of people who are willing to undertake such a journey. All of them applied to Mars One, an ambitious program to select and train the first human colonists to live out their lives on Mars. There are many reasons why this would have to be a one-way mission, but the short version is that by the time humans get to Mars, there would be no way they could survive a return to Earth. Mars’ gravity is less than ½ of Earth’s. Even if we could somehow simulate that environment on the journey to/from Mars, their muscular/skeletal structures would atrophy far too much to make survival in Earth’s 1g environment possible. That’s why the trip would have to be one-way: permanent exile on Mars.
And yet, for these people, such exile would give their lives tremendous purpose, one far different from those of us who would remain behind on Earth. It’s important to consider this because it shows that in a very real sense, humanity is going to have to change in a fundamental way if we ever become a true multi-world species.
This reminds me of John Haldeman’s Marsbound. Colonists were going to Mars and it turns out that they didn’t want to return. It gets weird after that, but it is science fiction. I’m not sure what purpose it would be to have a Mars colony unless it could somehow be terrascaped, but I’m sure someone would come up with a reason. We need to do a lot more with robots first.
I think the overall purpose would probably be “because.” There is something in our being that compels us to explore and to attempt that which has never been attempted before. In the very long term, humanity’s survival will depend on our ability to live on other worlds. But from a straight science perspective, I agree that we can get far more bang for the buck with robotic missions.