It's an advantage to have a top coach in your corner during a match. They can quickly pinpoint weaknesses of an opponent and come up with tactics that will maximize your chances of winning. They are also good at helping you get into the right mentality for a match.
Most players will never have a coach in their corner for most of their matches. But guess what? You don't need a coach in every match, not if you learn from the times that you do. It's still an advantage to have one, but the more you learn from a coach in the matches that he does coach, the better you become in matches where you don't have one.
Every coach is different, but in general, they stress some of the same things: what serves and receives to use, what shots to use (including shots to set up other shots), placement, and so on. But when you have a coach stressing these things in enough matches, it becomes second-nature for you to start looking for the same things.
If you have a coach in ten matches, and each time he's saying some variation of, "Attack the middle and wide forehand," "attack the middle and wide backhand," "attack the wide corners," "attack all three spots," then pretty soon it's ingrained on you to look for which of these placements to go for against various opponents, keeping in mind that each placement sets up other placements. (For example, attacking the middle often sets you up to attack the wide corner.)
Similarly, after ten matches with a coach, you'll start to get an idea for what types of serves and receives to use against different players, and it becomes ingrained to look for which ones to use, in particular what spins, placements, and depths. You'll also get an idea of what's the best mentality to have in a match.
And then, guess what? You'll play tournaments without a coach, and do really well. When that happens, some think, "See, I don't need a coach!" Others realize that the reason they did so well without a coach is because they learned from a coach in the matches where they were coached. And from then on, they'll continue to do well whether they have a coach or not - but it always helps to have a match coach, both for that particular match and as a continued "tune-up" to keep your tactical and mental skills at their maximum.
It's no different than any other type of table tennis training. It helps to have a coach work with you when you practice, but that doesn't mean you can't practice without a coach. Similarly, it helps to have a coach in a match, but that doesn't mean you can't play well without a coach. And once you've worked with a match coach enough, then if that coach has done a good job, you'll always have a coach in your future matches - yourself!