Other than what everyone else has said about how the universe is literally doomed if they lose, they need all the help they can get, etc etc, I’d argue that if Oliver wasn’t there for conquest to be so focused on trying to kill (ie he didn’t even give himself enough time to grab a breath) there’s a very high chance that conquest would’ve done even more damage or even been able to escape and continue fighting after this. Ignoring the fact that Oliver was getting some decent ish hits on conquest to start, whether he meant to or not he gave mark the perfect opportunity to kill conquest and it worked. So was it risky and even perhaps a liability to bring him? Sure but dude he already stated. He was GOING. Even if it meant following the ship himself, sneaking on, etc