It's an advantage to have a good coach in your corner. They can talk to you between games and during breaks and help you win, both tactically and with sports psychology. They can even do so during the games, often calling out (or whispering) winning advice. But they are often both over- and under-valued. How is this?
A coach can only guide you in a match, he cannot do your shots for you. In the heat of a point, he cannot stop play and tell you what your next shot should be. These are things you have to do on your own. A coach can suggest tactics that put you on a winning arc - but you have to execute those tactics or they won't help. A coach can use sports psychology to put you in a winning frame of mind - but that won't help if you can't execute the shots at a high enough level to beat a given opponent. The large majority of tournament matches aren't particularly competitive - just look at the RR results in a typical event and count the percentage of upsets. But in those matches that are competitive, a coach can sometimes make a difference.
But there's a missing part to this equation that many miss - the most important part. A coach may help in a given match. But to a player with a learning attitude, a good match coach does much more - he teaches you what types of things you should be doing and thinking about in a match. It may take time to absorb it, but if you are willing to learn, the long-term impact of a good match coach can be far greater than anything he might do in a single match. (Unless, of course, that match is for a big championship!)
For example, the players I coach can probably recite many of the things I'm regularly telling them between games: Attack the middle. Use the whole table. Focus on service placement and depth. What type of serves and receives to focus on. What are your go-to serves. How aggressive to play. And so on. But after a time, I no longer need to emphasize these things - because it becomes second-nature to these players!
A player I regularly coach in tournaments had a great tournament recently where he had no coach. When I asked him about it, he told me the tactics he had used. And he had a revelation - those were precisely the tactics I'd been harping on when I'd coached him before! Now, when I coach him, we simply need to focus on identifying things we are already looking to identify. He's learned to think tactically - which means he's learned how to win.