Extraodrinary Ministers of Holy Communion

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. (Redemptionis Sacramentum, 151)

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. (General Instruction of the Roman Missal, 28)