A one-on-one class can only ever teach you what you already know to ask about. In a group, you learn from everyone else in the room — the question another parent asks that you'd never have thought of, the worry someone else says out loud that you've been carrying quietly, the answer that turns out to matter more to you than it did to them.
You also get to see it, not just hear it. Comfort measures and labour positions land differently when you watch another couple try them, adjust, and get it wrong before they get it right. You pick up things by watching that no handout or private session can hand you.
And you leave with people. Everyone in the room is due within weeks of you, going through the same thing at the same time. Birth and parenting knowledge used to pass down through generations, and modern life has diluted that. This is how we rebuild the village — a small group of people who know exactly where you are, because they're there too.