The Code of Canon Law provides the rationale as to why Extraordinary Ministers of Holy Communion may be needed:
Can. 230 §3 - When the need of the Church warrants it and ministers are lacking, lay persons, even if they are not lectors or acolytes, can also supply certain of their duties, namely, to exercise the ministry of the word, to preside over liturgical prayers, to confer baptism, and to distribute holy communion, according to the prescripts of the law.
Can. 910 §1 - The ordinary minister of Holy Communion is a bishop, presbyter, or deacon.
§2 - The extraordinary minister of Holy Communion is an acolyte or another member of the Christian faithful designated according to the norm of can. 230 §3.
The 2004 Instruction on many elements of the Mass, Redemptionis Sacramentum, written by Pope St. John Paul II expands upon this, noting that it is not normative to need extraordinary ministers:
Only out of true necessity is there to be recourse to the assistance of extraordinary ministers in the celebration of the Liturgy. Such recourse is not intended for the sake of a fuller participation of the laity but rather, by its very nature, is supplementary and provisional. Furthermore, when recourse is had out of necessity to the functions of extraordinary ministers, special urgent prayers of intercession should be multiplied that the Lord may soon send a Priest for the service of the community and raise up an abundance of vocations to sacred Orders.
The universal liturgical practice is contained in the first part of The Roman Missal notes especially the need for good formation:
When the size of the congregation or the incapacity of the bishop, priest, or deacon requires it, the celebrant may be assisted by other bishops, priests, or deacons. If such ordinary ministers of Holy Communion are not present, "the priest may call upon extraordinary ministers to assist him, i.e., formally instituted acolytes or even some of the faithful who have been commissioned according to the prescribed rite. In case of necessity, the priest may also commission suitable members of the faithful for the occasion.”
Extraordinary ministers of Holy Communion should receive sufficient spiritual, theological, and practical preparation to fulfill their role with knowledge and reverence. When recourse is had to Extraordinary Minister of Holy Communion, especially in the distribution of Holy Communion under both kinds, their number should not be increased beyond what is required for the orderly and reverent distribution of the Body and Blood of the Lord. In all matters such Extraordinary Ministers of Holy Communion should follow the guidance of the diocesan bishop.
The ordinary minister of Holy Communion is a bishop, priest, or deacon. Properly understood, they are the only Eucharistic ministers present at Mass, since they are principally the ones to confect and handle the Sacred Species.
This function is to be understood strictly according to the name by which it is known, that is to say, that of extraordinary minister of Holy Communion, and not “special minister of Holy Communion” nor “extraordinary minister of the Eucharist” nor “special minister of the Eucharist”, by which names the meaning of this function is unnecessarily and improperly broadened.
While it has been popular practice, and in some parishes the term is still used as a shorthand to speak of those who will assist with the distribution of Holy Communion, we will, to the best of our ability, begin to use “Extraordinary Minister” to properly identify those who serve in this role. Using right language preserves the dignity of the sacramental nature of the priesthood, which can foster needed vocations, as well as make clear that this role is fulfilled by the laity only because of a lack of priests.
To be an Extraordinary Minister of Holy Communion you must:
The Extraordinary Minister of Holy Communion, by offering him or herself to this ministry of the parish willfully becomes a “public person”. In much the same way that a priest is never out of the public eye, an extraordinary minister is oft evaluated when they are not in church. The role of Extraordinary Minister may be reserved to the Liturgy or when bringing Holy Communion to the Sick, but it is a title that people associate with a person even when Mass is not taking place. As such, the Extraordinary Minister ought to reflect by way of life the Eucharistic Mystery one is privileged to hold in his or her hands.