Our universe is massive. It is so massive, that we will never truly comprehend how massive it is. And might I say again, MASSIVE!!!
All joking aside, there are roughly two discoveries waiting for us in that massive universe that are more important than any others: that we are not alone and that we are alone. Either or would be a great game changer that would force us to question our place in the cosmos.
If we ever find even the simplest forms of life, and we can prove that they did not originate from Earth, then we know that there will be more out there. And if live can evolve in multiple places in multiple forms, then so can sentience. If we find the most basic microbes on Mars, Jupiter's Europa, or Saturn's Titan, we can expect to find it on many other worlds in an unimaginable number of forms.
It would actually be more difficult to prove that there is no other life. Even if we scour our entire solar system and find nothing, we can only conclude that it takes a very special set of requirements for life to form, potentially requiring a planet with the right mass, in the right orbital distance, and orbiting the right kind of star. We could then search other places that fit this criteria, find nothing, and can then only narrow the criteria even more. It would take a dedicated search of the entire galaxy before would could conclude that there isn't any other life out there.
We'll call these hypotheses A and B. Of the two, A, that life is out there in some form or another, is very likely correct. In the past century, we have found life existing in the more remote and inhospitable of place, such as Antarctica, a mile below the surface of the Earth, and near volcanic vents on the ocean's floor. The real question arises in what it takes for life, as we currently define it, to begin at all. After all, the life found in these places probably did not start there, but developed elsewhere.
One requirement for life might be the existence of an atmosphere. In out own solar system, we know Mercury, Venus, Earth, Mars, Jupiter, Jupiter's four largest moons, Saturn, two of Saturn's moons, Uranus and Titania, Neptune and Triton, and Pluto, thus far, have been discovered to have atmospheres. One postulated requirement for life is liquid water. So far, only Earth has been conclusively proven to have liquid water. However, water ice has been discovered throughout the solar system, including even Mercury. Saturn's moon Enceladus has polar 'cryovolcanoes' that shoot out water vapor. Jupiter's Europa has an icy surface, and could have liquid water underneath, because of tidal forces from Jupiter. And perhaps other liquids would suffice. Yet another possible requirement for creating life could be the existence of magnetic fields, to hold off radiation. Four planets and one moon have been shown to have magnetic fields.
So what if we do find life? Even finding bacteria-like organisms would tell us that life has developed off of Earth. That life would then be bound to evolution, thus giving it the opportunity to grow in complexity, possibly beyond humanity. Even the thought of such beings, without even meeting them, would force us to reflect on our very existence and purpose.
And if we never find life? Humanity would be free to terraform any other bodies without regards to environmental impact. We would never question that this is humanity's universe. We would forever be alone.
I, for one, hope we do find other life. I hope we're forced to grow up as a species. I don't want humanity to have a perpetual adolescence.