John's words point to the vivid recollection of the person of Jesus that John still had in his old age. For John those memories were permanently etched on his mind as if the events had just happened. He uses terms such as "we have heard" "we have seen" "we have looked upon" and "our hands have handled" that strongly affirm the physical reality of the incarnation of Jesus.
John's personal testimony refutes the false teachers who boasted arrogantly and wrongly about the Christ they had never seen or known. John wanted his readers to be partakers with him in possessing eternal life. We can have fellowship with the Father and with His Son Jesus Christ, which ultimately produces a fullness of joy.