Purpose number one: To confirm the teaching of the Apostles.
Purpose number two: To build up the Church.
Purpose Number One
First off, we must remember that the Apostles themselves are confirmation of the truth that they preached. Remember, an Apostle, according to Acts 1, is someone who had been an eyewitness of the resurrected Christ and had been commissioned by Him. Paul gives us this idea as he defends his own apostleship in Galatians:
Paul, an apostle--not from men nor through man, but through Jesus Christ and God the Father, who raised him from the dead-- (Gal 1:1)We also have John who testifies as an eyewitness of the resurrected Christ in I John:
For I would have you know, brothers, that the gospel that was preached by me is not man's gospel. For I did not receive it from any man, nor was I taught it, but I received it through a revelation of Jesus Christ. (Gal 1:11-12)
That which was from the beginning, which we have heard, which we have seen with our eyes, which we looked upon and have touched with our hands, concerning the word of life-- the life was made manifest, and we have seen it, and testify to it and proclaim to you the eternal life, which was with the Father and was made manifest to us-- that which we have seen and heard we proclaim also to you, so that you too may have fellowship with us; and indeed our fellowship is with the Father and with his Son Jesus Christ. (1Jo 1:1-3)This, of course, is not something that originates with the Apostles. The Old Testament Prophets as well were confirmed by God through the working of miracles. And Jesus was also "...a man attested to you by God with mighty works and wonders and signs that God did through him..." (Acts 2:22)
This is the first purpose of the gifts.
Purpose Number Two
The gifts were also given to build up, edify, or mature the body of Christ, the Church. This is most evident from Paul in Ephesians:
And he gave the apostles, the prophets, the evangelists, the pastors and teachers, to equip the saints for the work of ministry, for building up the body of Christ, until we all attain to the unity of the faith and of the knowledge of the Son of God, to mature manhood, to the measure of the stature of the fullness of Christ, so that we may no longer be children, tossed to and fro by the waves and carried about by every wind of doctrine, by human cunning, by craftiness in deceitful schemes. Rather, speaking the truth in love, we are to grow up in every way into him who is the head, into Christ, from whom the whole body, joined and held together by every joint with which it is equipped, when each part is working properly, makes the body grow so that it builds itself up in love. (Eph 4:11-16)I want to particularly point out here that Apostles and Prophets are not only included in this 'edifying' function of the gifts, but are also two of the main gifts in view. So our interpretation of a continuing or ceasing of the gifts hinges on this passage, as the good Dr. Moorhead has pointed out in his series on the gifts (start here). It has truly got to be all of the gifts or none of the gifts. There cannot be any middle road, logically speaking. If we still need the Pastors and Evangelists, we still need the Apostles and Prophets.
It is my belief that these verses clearly tell us that we need these gifts, and that we will need them until we are a mature and perfect body. That will happen at the return of Christ.