We had to start somewhere. In trying to determine where life might likely arise in other parts of the universe, a guide and yardstick had to be set. Where else should we begin with but home? Such is what i think is behind the concept of the Habitable Zone (HZ), a region of space where conditions are favorable for life as it may be found on Earth. More specifically, HZ is a region around a star in which water can exist in liquid form.
Its very anthropomorphic, I know, and is constrained for water-based and carbon-based lifeforms, but it's a very good start in the search for exoplanets and life in deep space.
Probabilities and the likelihood of finding life in other places begins with Earth as the model and guide, hinging on Water. Planetary missions, such as the Mars Exploration are on the lookout for signs of water. Because water means life, er to begin with.
Slowly but surely, we are now expanding our sphere of thought from the basic Habitable Zone concept to other worlds. From Mars, Jupiter and its moon Europa, all the way to an exoplanet called Gliese 581c. Soon there will be more worlds to think about. And more platforms of life to speculate on, not just carbon, but perhaps methane.
And the day will come when a variety of Habitable Zones will be identified, not just the traditional Earth-like or water-based kind that we know of.
When we finally find signs of life or life itself on a distant exoplanet, we will remember the day when the concept of the most basic Habitable Zone was formulated...hey, we had to start somewhere.