Read through John 16:12 – 15. To be fair you need to look at the entire context. This is just to the right of the middle of the most important section of John’s gospel. The 21 chapters cover about 3 years of Christ’s life. Chapters 13 – 17 cover about 6 hours. So based on the percentage of space that John devoted to those 6 hours, that section is significantly important. It was the final teaching to the disciples before Christ’s arrest, trial, and crucifixion. Last words are important. Especially the last words of the Lord.
In 14 and 16 the Lord focuses on the ministry of the Holy Spirit. Note that John 16:12 – 15 is the last thing He says to them about the Spirit before He is arrested.
Based on what Christ says about the Spirit here what would one who is filled with the Spirit want to talk about?