Why was it necessary to "restore" the innovative third century practice of Confirmation?
Confirmation did not exist nor is it even mentioned in the New Testament. Confirmation is a practice that "naturally followed from infant baptism to give the opportunity for the candidate's conscious participation and commitment." (20 Centuries of Christianity, A Concise History, Hutchinson and Garrison, 1959, Harcourt, Brace & World, Inc., New York, p.146.)
The Bible Dictionary from the 1979 LDS edition of the scriptures under Confirmation (p. 649) tells us that the word "confirmation" is not found in the N.T. though the rite is found in many places e.g. Acts 8:14-17. What the N.T. describes, however, is not the rite of confirmation but the "ordinance" of being given the gift of the Holy Ghost.
The name itself, confirmation, denotes a confirming or acceptance of one's previously administered baptism. Confirmation was developed to complete the initiatory act of baptism and thus become a full member of Christ's church. The Eastern church never separated the time between baptism and the receiving of the Holy Ghost and therefore never developed a rite for Confirmation. The gift of the Holy Ghost is bestowed after baptism and is called chrismation because the baptized is anointed with chrism (consecrated oil) as the baptized receives the gift of the Holy Ghost.
A search of the LDS Collectors Library '97 results in many quotes that show that what is commonly called "confirmation" today is actually two actions in a single ordinance. Edward C. Rich, Conference Report, April 1918, p. 125 tells us that confirmation includes acceptance "into the Church and the reception of the Holy Ghost by the laying on of hands."(italics added) B.H. Roberts in his Outline of Ecclesiastical History, p. 324 tells us that "Confirmation into the church follows baptism and is performed by the laying on of hands, by those who have authority in the church. The Holy Ghost is imparted in the same manner and at the same time--one ordinance." Delbert L Stapley, Conference Report, October 1965, p. 14 in speaking of ordinances included: "(1) baptism by immersion for the remission of sins, (2) the laying on of hands for both confirmation and the bestowing of the gift of the Holy Ghost, (3) ..." (italics added). These are just a few of the references that show that two separate actions have been combined into one ordinance.
In the Catechism of the Catholic Church, 1992, pp.326-333, confirmation is described as a ratification of Baptism, thus completing Christian initiation. Through confirmation, the candidate is sealed with the Gift of the Holy Spirit. Confirmation is one of the church's seven sacraments. "Every baptized person not yet confirmed can and should receive the sacrament of Confirmation. Since Baptism, Confirmation, and Eucharist form a unity, it follows that 'the faithful are obliged to receive this sacrament at the appropriate time', for without Confirmation and Eucharist, Baptism is certainly valid and efficacious, but Christian initiation remains incomplete." (p. 331). "The age of discretion" is used in the Latin tradition for the age at which a person should receive confirmation. Confirmation has sometimes been referred to as the "sacrament of Christian maturity."
It is puzzling that the LDS have adopted the Roman Catholic (Western) name and practice of confirmation since it takes the Eastern church's approach to the bestowal of the gift of the Holy Ghost shortly after baptism and therefore nothing remains to be confirmed.