It is important to remember that Matthew was writing his gospel for a distinctively Jewish audience. He spends a great deal of his gospel showing how Christ fulfilled the prophesies of the Old Testament. One of the chief objections by the Jewish peoples was that Jesus had not ushered in the Day of the Lord, instead he died on a cross. Matthew, when he wrote his gospel sometime after 50 CE, likely wanted to instill the thought that Christ had indeed ushered in the Day of the Lord, even though it was a thoroughly spiritual kingdom. In so doing he may well have either included details no other gospel writer was interested in including, or else he may have created the account to support the Day of the Lord motif in the Old Testament.
Most scholars agree the historicity of such an event is unlikely.
Calvin, in his Institutes claimed the event was included to demonstrate that those saints predating Jesus Christ were included in the realm of Christendom. But before we dismiss the tearing of the Temple curtain, the earthquake, and the resurrection completely, there is one interesting tidbit to consider. Interestingly enough, the crucifixion of Christ took place 40 years before the fall of Jerusalem as well. If this is historical, perhaps. . . . If these saints actually were resurrected, in the end their fate would have been the same as Lazarus who, tradition says, died at a ripe old age.