Why were some people brought out of their graves immediately after Jesus' resurrection?
They were brought out immediately after His resurrection to demonstrate the power of resurrection and to fulfill the Old Testament first-fruits pattern. Unlike Lazarus, who was merely revived to die again, these were truly resurrected, entered the Holy City, and appeared to many.
Les explains that previous raisings, such as Elijah and the widow's son or Lazarus, were not resurrection; they were merely bringing someone back to life only to die again. Resurrection began with Christ's own resurrection, which opened the door to the whole doctrine of life after death and had to begin with the Creator Himself. Immediately after His resurrection, to show its power and to fulfill first fruits as practiced in the Old Testament economy, these people were brought out of graves around Jerusalem, went into the Holy City, and appeared to many. Since they were resurrected, not just raised to die again, Les concludes they had to go on into glory, thus fulfilling first fruits.