It is probably within the reach of human technology to make the climate of Mars inhabitable  It might be several centuries before people could live there, even with breathing apparatuses, but some of the world' s great temples and cathedrals took centuries to build  Research efforts now are justified if there is even a chance of making another planet inhabitable
Besides, the intellectual exercise of understanding how the Martian atmosphere might be changed could help in understanding atmospheric changes inadvertently triggered by human activity on Earth
efforts to change the climate of Mars could facilitate understanding of the Earth's climate