Since it was the day of Preparation, and so that the bodies would not remain on the cross on the Sabbath (for that Sabbath was a high day), the Jews asked Pilate that their legs might be broken and that they might be taken away.  So the soldiers came and broke the legs of the first, and of the other who had been crucified with him.  But when they came to Jesus and saw that he was already dead, they did not break his legs.  But one of the soldiers pierced his side with a spear, and at once there came out blood and water.  He who saw it has borne witness—his testimony is true, and he knows that he is telling the truth—that you also may believe.  For these things took place that the Scripture might be fulfilled: “Not one of his bones will be broken.”  And again another Scripture says, “They will look on him whom they have pierced.”
At the Passover, every Jewish family would sacrifice a lamb in the evening. They had to spread the blood of that lamb over the doorway of their house, and then they cooked the meat and ate it. The lamb had to be a good one. It couldn’t be injured, diseased, etc. Those lambs could be cooked and eaten any other day. For the sacrifice of the Passover, though, they had to be one in good shape.
If a criminal made it to the cross alive, they typically stayed up there for days. Dehydration, bugs, vultures, and raw sunlight would take their toll and cause a person to die within a few days at the most. The Romans had perfected the art of crucifixion so criminals would hang there no matter what shape they were in by the time they were hung up. They also hung them up in all kinds of positions, so as to increase the suffering and shame of a dying person.
The fact that Jesus was already dead when they checked on Him shows the brutality of what He went through before He was hung on the cross. Even with all of that abuse from the world, He was still perfect. He was still unbroken and fulfilled His calling just the way He was supposed to.