This is basic carrot and stick diplomacy. Announcing that aid is being cut is the stick (i.e. the punishment). United States Secretary of State Mike Pompeo can now offer the carrot (the reward).
We may not be publicly told what the possible carrots are. An obvious possibility is that the aid might be restored if Pakistan does something or another. Of course, we may not be told what the US wants Pakistan to do.
It is not at all uncommon for diplomatic discussions to be private. That allows both sides to offer things that might be publicly embarrassing if known. If the negotiations are private, then they only need to publicly share those things on which they agree to publicly visible changes. They don't need to share that they considered other things.
Another issue is that they may want to announce the aid cut now so as to allow the public reaction to settle before the Pompeo visit. If they cut the aid after or during the visit, then public reaction may make it harder for the Pakistani government to announce a new agreement. This is because people will tend to forget the benefits of the new agreement when presented with the negatives of the cut in aid. If they announce the cut in aid first, then they can time the announcement of a new agreement for when the public is most receptive. The earlier the aid cut is announced, the sooner that point in time is reached.