1 john 1:2 this one who is life from God was shown to us, and we have seen him. And now we testify and announce to you that he is the one who is eternal life. He was with the Father, and then he was shown to us.
John is reenforcing that Jesus came and that he was seen. That he still lives and we need to spread his word. That you will receive eternal life if you choose to believe. Again reenforcing that Jesus was with God then he was sent to earth and he was seen.
This verse is important to remind us that Jesus was seen by many; many recount his life. It isn't a fictional story that is passed down. It is the actual accounts from many who saw him.
As a disclaimer, I am not a Christian, and I should also say that there are likely others besides my own interpretation, but I take it to mean that it was John's belief that Jesus was the material proof of the immortality promised by God.