Artist's rendering of the alien planet GJ 667Cc, which is located in what could well be the habitable zone of its parent sun in a triple-star system.
Here are a couple important points about this particular system. First, the planets orbit an M dwarf star. M dwarfs provide very poor environments for life. They show erratic brightness fluctuations, and they produce powerful flares with dangerous radiation. Planets in the habitable zone of an M dwarf will spin down fairly quickly, leading to a "tidally-locked" situation that leads to all sorts of problems.Some astronomers are eager to discover planets that can support life because if they do it'll be an important step in discrediting the modern argument for an Intelligent Designer. As it is there's no reason to doubt that our planet is unique, perhaps not just in our galaxy, but in the universe. Books like Rare Earth and Privileged Planet make this case pretty convincingly by identifying a raft of characteristics any life supporting planet (and its star and galaxy) must possess.
Second, terrestrial planets more massive than Earth are likely less habitable than Earth for several reasons. For instance, they will have less surface relief, which makes it less likely they will have dry land.
These books support the claim - inadvertently, perhaps, in the case of Rare Earth - that the earth is extraordinarily special, that in some sense we really are at the center of the universe, at least ontologically.
If it were discovered that our planet is not privileged, however, that it's not special, not unique, then the design argument is rendered a little less compelling. Thus the search for other suitable planets is the "Holy Grail of exoplanet research," not only for those astronomers curious to learn as much as they can about the cosmos and life, but also of those astronomers seeking, for whatever reason, to discredit the belief that the earth is a very special place.