Justin was the first to discover Zechariah 12:10 but he used it only to prove that the Jews pierced only hands and foot of Jesus (being condamned to see the traces of the crime in the day of Parusia). He didn't use it to prove that the Jews pierced the side of Jesus.
Therefore only after Justin, the story of the piercing of the side of Jesus was interpolated in the fourth gospel.
This hypothesis finds a remarkable evidence in the fact that both John and the Apology 52:12 have the same words in Greek:
opsontaï eïs hon exekentêsan
A text that differs very a lot from the Greek of the septuaginta.
The piercing of the side of Jesus serves to prove, against Marcion, that the body of Jesus was not ethereal, since from it water and blood go out. The blood is true human blood, since the presence of water mixed with it proves that that blood was able to be dried, like any human blood.
When I see someone who doesn't believe that proto-John is a marcionite gospel, I am 100% persuaded that I have before me a Christian apologist.