Into All Truth
The Holy Spirit was given to prepare the disciples for revelation at a whole new level. He would take them where Jesus couldn't. Perhaps this is part of the reason Jesus said,
Nevertheless I tell you the truth. It is to your advantage that I go away; for if I do not go away, the Helper will not come to you; but if I depart, I will send Him to you. John 16:7
The indwelling Holy Spirit enables us to bear more of the revelation of Jesus than was possible for the original twelve disciples.
However, when He, the Spirit of truth, has come, He will guide you into all truth; for He will not speak on His own authority, but whatever He hears He will speak; and He will tell you things to come. 14 He will glorify Me, for He will take of what is Mine and declare it to you. 15 All things that the Father has are Mine. Therefore I said that He will take of Mine and declare it to you. John 16:13-15
One of the assignments of the Holy Spirit is to let us know what is to come. If you read commentaries and various reference materials, you'll notice most think the promise of knowing what's coming is all about us being aware of coming calamities. Theologians tend to focus on problems because few truly believe in the glorious church. Everyone from world leaders to musicians, to actors and business leaders, are telling us of the coming calamities. We don't need the Holy Spirit for that purpose when people without God can do it. Rather, we need Him to see the coming glory! The warnings of difficulties are necessary as they help us keep our priorities straight. But it's the Father's good pleasure to give us the mysteries of the kingdom. And there's no pleasure in speaking of the death and destruction of the unrighteous. It's still called the good news for a reason.