The first public celebration of drawing baptized, but uncatechized adults into the community is the Rite of Welcoming (RCIA 411-433) described briefly here:
405. The period of preparation is made holy by means of liturgical celebrations. The first of these is a rite by which the adults are welcomed into the community and acknowledge themselves to be part of it because they have already been marked with the seal of baptism.
406. Once a rite of reception has been celebrated, these adults take part in celebrations of the word of God, both those of the entire Christian assembly and those celebrations arranged specially for the benefit of the catechumens (see RCIA 81-84).
Two interesting conclusions can be drawn from these sections. First, an answer to the question of why wouldn’t a pastor dispense with public liturgies and catechize newcomers individually or delegate this task. The simple reason is that the process with ritual is how it all is “made holy.” The object is not just the imparting of information, but to cooperate with the sanctification, not only of newcomers, but also of the parish community.
Second, being schooled by the Word of God is just as important for the baptized as it is for the unbaptized. We might ask why the Word isn’t more prevalent in our own prayer and ongoing catechesis. And maybe in communities with a well-grounded catechumenate, that question is asked and the Word encountered more deeply.