Although we come to Christ individually, that we do not live in Him solitarily, so that we have Christ, who is the shepherd of the sheep, that He is the bridegroom. And He is, if we think in architectural terms, He is both the link pin and the apex of the entire building, which is His body, His church throughout the entire world. 

It's really just making the point in a way that would be immediately understandable to the people of the time, that as they looked at the temple structure and realized just how crucial the capstone was, and Jesus says in this... Peter says, "Now, this is exactly who and what Jesus is."

He is crucial to this whole building in the same way that Paul, when he uses the metaphor of the body, he says Christ is the head. People understand that. That's a very important part of it.