The idea of a 'habitable' planet is a cool one, but how relevant is it? Do we really expect that climate is going to be a concern by the time we are colonizing planets 600 light years away?
Now that I think about it, I guess it could be that they are looking for life on other planets and figure that alien life that is recognizable to us will likely be on planets in this zone. I looked at Kepler's mission and they don't really specify why they are looking:
"The Kepler Mission, NASA Discovery mission #10, is specifically designed to survey our region of the Milky Way galaxy to discover hundreds of Earth-size and smaller planets in or near the habitable zone→ and determine the fraction of the hundreds of billions of stars in our galaxy that might have such planets."
Well, one of the reasons to look is certainly to figure out what kinds of solar systems are typical. Are all solar systems like our solar system? What are the differences? How common are rocky planets? Its mission is to give us a more detailed picture of the solar systems out there.
It’s about knowledge, not whether or not that knowledge is actually useful.
Now that I think about it, I guess it could be that they are looking for life on other planets and figure that alien life that is recognizable to us will likely be on planets in this zone. I looked at Kepler's mission and they don't really specify why they are looking:
"The Kepler Mission, NASA Discovery mission #10, is specifically designed to survey our region of the Milky Way galaxy to discover hundreds of Earth-size and smaller planets in or near the habitable zone→ and determine the fraction of the hundreds of billions of stars in our galaxy that might have such planets."