"Therefore many other signs Jesus also performed in the presence of the disciples, which are not written in this book; but these have been written so that you may believe that Jesus is the Christ, the Son of God; and that believing you may have life in His name."
Imagine all that Jesus' disciples remembered and recalled about their time following Jesus. There had to be numerous stories and teachings, things that happened on their journey. John also tells us in the next chapter that if all which Jesus did were written in detail, the world itself could not contain the books (21:25).
But the holy Spirit has led them to record specific accounts and John tells us why: so that we may believe Jesus is the Christ and by believing we may have life. That is the purpose for the Gospels. They are not simply history lessons, or rule books. They are the Word of God that tell us about true events in order to reveal that Jesus is the Christ. That Jesus is the Savior of the world. This truth should drive us to follow Him and receive life that only He can give.
This also lets us know that what we have in Scripture is sufficient; it is enough! We don't need more signs, more words, or God would have included more. We don't need to add anything at all and we should not seek such things. He has given us everything we need to have life through His Word.
Does God still work today? Yes! Can we still see evidence of God working in the world and in people's lives? Absolutely. But we only know something is from God because of His Word. The only reason we know who God is, is because He loves us and has chosen to reveal Himself so we would seek Him. If it was His desire to be known by His creation, why would He withhold anything needed to do so? He hasn't! He has revealed Himself through His Son and through His Word.
We should never seek after more than what God has promised is enough. We should never seek after or follow those who tell us that more is necessary and that they can reveal it, or that God has only revealed it to them. And we should certainly never base our faith on someone's experience or revelation if it is at odds with Scripture. To do so disregards the authority of the true Word given by God and confirmed by the resurrection of His Son!