Well, the thing that sets living stuff apart from non-living seems to be the ability to sustain life (usually in the form of offspring). From this we can infer the purpose of life to be keeping life alive.
Spreading live across the universe being a good way to keep some of it going seems plausible. I'd obviously prefer intelligent beings like humans doing so, since so far evolution indicates this development path to have the best chances long term. But if not possible or too costly, spreading the next closest thing to us that is possible makes sense. At least as a form of backup. And sending microbes seem quite easy...
Well, that is obvious. Because causality works backwards and because we need conscious observers everywhere to make sure the laws of nature turn out just right for the universe to exist.
I didn't claim either of those things; if your counter-point is something better than "hurr durr humans are a cancer we deserve to die, we ruin everything" then I'd be interested to hear it. But if that is your counter, then I don't want to hear it.
But, the universe has been getting more complex all the time - every stage of its history over billions of years has moved towards more things, more complex interactions, more abstract organizations - fundamental forces splitting off, plasmas, galaxies and stars, fusion and heavy elements, solids and liquids and planetary systems, chemistry tectonics and weather systems and day/light cycles, LIFE, multi-celled life, biological systems and their interaction with weather, communication, socialization, organization, co-operation, industrialization, it just gets more abstract and more involved.
Unless you think a God exists and put a barren radiated rock somewhere for a good reason, then you may as well conclude that the universe is trying to get more complex. Like you can say that humans moved grass over the ocean for our own benefit, you can also interpret it that grass got humans to move it over the ocean for grass's long term survival benefit. There is only one known life-o-sphere and if one-day it is going to outside Earth, it needs a highly intelligent species to appear and move all the rest of it. That could be us, it could be why it looks like we're screwing up the planet - because long term, the point is not for us to stay here as bucolic agriculturalists, but to be an ignition chamber for something (probably silicon) to burst out and go forth.
What makes it our job is that nobody else can. Whales cannot do it. Cows cannot do it. Oak trees and crickets and fungus and cockroaches and E.coli aren't going to. Only humans can escape Earth intentionally, so humans have the obligation to do that on behalf of all life.
Compare how many humans think life is important (a lot), with how many humans think it important that Mercury stay barren until it gets swallowed by the Sun at the end of its fuel supply (the number of people who have even considered the idea in all human history, rounds to zero).
What makes it important? WE make it important, because - in the absense of a deity giving orders to us - who else can give meaning to anything, except humans? If nothing else, a more populated universe would be more interesting for us. Eventually.
