John 13:19 Now I tell you before it come, that, when it is come to pass, ye may believe that I am he.
Jesus told them before it happened so that they would know that He was God. If He knew before it happened then He was no ordinary man. Jesus came to this earth with a purpose. He knew exactly what was going on.
He knew that they might not understand what He was saying as it happened but when it was over they would remember that He had told them everything.
They needed to know that Jesus was no ordinary man. He was no ordinary teacher. He was God in human flesh.
He told them before so that they would finally realize the truth. They would believe in Him. They would believe that He was who He said that He was!