There's nothing else that can bring people together more quickly than a crisis. It seems that in times of great need, people forget their little squabbles and can unite to help one another. So what better way is there towards international cooperation then a joint global effort to overcome some of the obstacles faced getting to Mars? It's something everyone on Earth could feel proud of achieving and not just for doing it, but for joining together to accomplish it. And why do we need international cooperation? Because it's the forerunner to international peace.
Is there anything out there? Everyone must be interested in the answer to that question. Mars may not be the best place to look for life, but it is one of Earths closet earth-like neighbour, and one we can get to in the near future. The debate oo life on Mars is one of the biggest mysteries surrounding the planet, but certainly not the only scientific reason for going. Mars, in a lot of ways, is very similar to Earth, and discovering Mars's past could lead to a better understanding of our own planet and its possible future. The most effective way of answering this is to go there in person.
Advancement of species
To overcome a problem that will lead to something unique that civilisation has never had the opportunity to achieve before. And a new place to move in too, what could be better.
Development of Technologies
Advancement of Technologies has never taken a giant leap more quickly than when humans set out to kill others - our greatest achievements were accomplished during some of our darkest hours. Just think what we could develope if we could do this in peace time.
To see what's there
To the best of our knowledge Mars has the biggest volcano in the Solar System, dwarfing anything Earth has by miles (literally). It has canyons miles deep, vertical walls reaching into the skies and a history that has seen its seas dry up, its atmosphere disappear and a hundred mile wide asteroid thunder into its side. Sounds like an exciting "tourist destination" to me.