Throughout Jesus's life, Jesus has done many feats, from water-to-wine to healings to raising people from the dead, including himself. The disciples (and a few others) got to see those feats. But Jesus did way more feats than those listed in John's recording. Some of the other feats that Jesus did are written in the other three recordings of Jesus's life -- Matthew, Mark, Luke.
But the reason why John puts some of those feats inside is that the reader "may believe that Jesus is the Christ, the Son of God" and that by believing, the reader "may have life in His name" (20:31). Jesus made a lot of promises, like how he provides us a way to be "born again" despite screwing up and how he provides us peace, the ultimate Helper, and a big calling. Way back in John 2:31, John talked about how Jesus proclaimed that he would get resurrected three days after the death, and John recorded in John 20 that it happened. Promises made, promises kept. Thus, John provides us the grounds of why we can believe in Jesus as Lord and Savior.
Believing in Jesus is essentially accepting that we need Lord Jesus (who is Son) to renew us and counting on Jesus and his resurrection for personally renewing us as Savior. That's it for us. Jesus as Lord (and Son) rules over death via the resurrection and Jesus as Savior does the saving itself. When we believe in Jesus, we are guaranteed everlasting life that would ultimately not end.
For the readers who decided to accept Jesus and count on Him for saving us, you are guaranteed eternal life. I don't have a "I raise my hand" button or "I accepted Jesus" button because faith starts internally. (You can raise your hand though.)