There are a lot of things that Jesus told the disciples, from the call to love and serve to the promise of the Holy Spirit. There are even more topics to be discussed, but Jesus told the disciples that "[they] cannot bear [the other topics] now" (16:12). At that time, the disciples were likely too focused on fear to understand Jesus -- especially with how physical separation from Jesus can actually be a good thing for the disciples.
However, Jesus once again reiterated on the promise of the Holy Spirit. In particular, Jesus gave some examples on what the Holy Spirit will do.
Guidance into all truth (16:13). In the particular context, the Holy Spirit will help the disciples more fully understand the other topics (including some things to come, especially upcoming persecutions) that Jesus was referring to (hence the use of "complete truth" in some translations). The Holy Spirit also guides by only using truth, too. But it's also heavily implied that the Holy Spirit eventually leads us into every truth regarding God.
Relaying information from God (16:13-15). The Holy Spirit doesn't work in isolation but in tandem with Jesus and Father. It's also likely where the Holy Spirit gets those truths.
Glorifies Jesus (16:14). This is because of what the Holy Spirit does.
Jesus likely tells these things as encouragement for the disciples to look forward to, even in difficult times. Today, we can seek the Holy Spirit during difficult times for guidance.