I would expect that peer pressure can make people stop doing evil things (either by force, or by changing their cost-benefit calculation of evil acts). Objective morality, or rather a definition of morality consistent within the group can help organize efficient peer pressure.

So in a conversation between a person A who believes in objective morality and a person B who does not, a possible motive for A is to convince onlookers by any means possible that objective morality exists. Convincing B is not particularly important, since effective peer pressure merely requires having enough people on board and not having any particular individual on board. In those conversations, I always had the role of B, and I assumed, perhaps mistakenly, that A's primary goal was to persuade me since A was talking to me. Thank you for the insight.