I believe it used to be that all families with small children could board with those needing more time. Then they changed it to only families with infants, with the families with older but still small children after zone 2.
They probably do the announcement this way to make sure those who were used to board early, or board early on other airlines, know that they no longer can board first. But I think that rule changed over a year ago, so maybe people should be used to it by now. I basically just see it as a reminder that they changed a policy.