Below is an excerpt from a really good article by Bob DeWaay:

“Jesus made the following promise as He prepared his disciples for His departure: “Truly, truly, I say to you, he who believes in Me, the works that I do shall he do also; and greater works than these shall he do; because I go to the Father” (John 14:12). Before we discuss the meaning of “works” and “greater works” in this verse, we should consider the significance of works in the Gospel of John. The previous verse tells us the key purpose of works: “Believe Me that I am in the Father, and the Father in Me; otherwise believe on account of the works themselves” (John 14:11). The works that Jesus performed were to lead us to faith in Him as being God incarnate. We are to believe that He is one in essence with the Father. Jesus states this elsewhere in John: “But the witness which I have is greater than that of John; for the works which the Father has given Me to accomplish, the very works that I do, bear witness of Me, that the Father has sent Me” (John 5:36). The works bear witness to the person and mission of Christ. This must be kept in mind as we contemplate the “works and greater works” of John 14:12.

. . .The ones who do the works are believers in general: “he who believes in Me.” This promise is not restricted to a special class of elite Christians or latter day apostles. This is an important consideration. The purpose of the works is to lead people to faith in Christ. Many mistakenly think that their purpose is to validate the person doing them. We are called to go to a special meeting to hear a great “miracle worker” and find relief from various afflictions. Testimonies of those who have been healed are used to promote the healer. This is not at all the purpose of signs and works of God in the Bible. The contention being made by Christ and His apostles was that He was God Incarnate, the promised Jewish Messiah, and that only He could bring us to the Father (John 14:6). John was called the greatest prophet (Matthew 11:9-11) yet he did no miraculous works (John 10:41). John bore witness to Christ through his preaching and fulfilled God’s purposes. It was John the Baptist who said: “He must increase, but I must decrease” (John 3:30). The purpose of the works was not to make great the fame and fortune of the prophet, but to bear witness to the person and work of Christ. . . .

The entire article can be found at Critical Issues Commentary. It is an excellent, Scripture based treatment of miracles, signs and wonders and their purpose.