It sacrificed its life energy to bring new life to all the planets it previously destroyed, and seemingly reformed every other planet it destroyed in the universe, and possibly multiverse.
They showed a star system being reformed and then they were teleported to the other...
So it could be said that that one Infinite doing doing that reversed the entire process, across the multiverse. Or at least across the universe.
It doesn't make sense that it sacrificed its life to do this, but it's still not a bad feat. A likely explanation is that it was touched by the heroes' speech and wanted to be a part of the whole life shit. And it sacrificing its life energy only had to do with actually giving new life to new beings as opposed to just reforming worlds.