If martians had built anything cool there we would have known by now. It's just not very likely there's much interesting up there unless you're interested in the little things.
Edit: The effect is so strong, I have occasionally wondered if it is a concerted effort to get more people to start Mars startups, like nerd sniping. "Hurr you can't go to Mars like ever!!" -> "....just watch me". lol.
We'd have actual proof that we are not the only life in the universe (even if just on Mars it may be microbial-like). Actual proof that we are not special or unique.
I view it as like when a small baby/toddler realises that other people exist and that they are not the sole focus of the world's attention - a major development milestone.
Religion, life's meaning, our place in the universe -- everyone will face an immediate reckoning here and have to reconcile their previous notions.
But even more impactful is that we'll start to be able to make much better approximations of Fermi's Paradox. It might suggest that the universe is teeming with life. Then questions about how it evolves and reaches sentience will move to the forefront. We'll have much better ideas.
We might start to get a little anxious about what's lurking out there, and we will probably start spending massively on space telescopes.
Society will start looking up.
I think there is life outside this planet, but we do not know for sure.
If we don't find life in this solar system, we quite possibly might never.
You probably won't live to see this being confirmed.
For these reasons, if we find life, it'll change everything.
/s