Immediate benefit of in-game coaching

For, say, a young 1600 level player, how much difference do you think a good coach can make during a tournament match, expressed in whatever terms make the most sense to you? I realize the more important benefits may show up over the long term, but for now I'm interested in the difference during that match/event/tournament.

It can make a huge difference - there's a reason essentially every top junior in the U.S. (and around the world) has a coach at all their matches. However, to maximize this, the coach needs to be familiar with the player, and the player must have confidence in the coach. The last chapter (four pages) of my book Table Tennis Tactics for Thinkers is about this. Here's an example from my November 29, 2016 blog - start with fourth paragraph under "North American Teams," which starts, "Does coaching make a difference? You better believe it!" A coach also should be able to teach the player to develop the habit of tactical thinking, a habit which will help them lifelong in table tennis and other endeavors as well.