...to know how to fight off break points, take advantage of break point opportunities, change tactics when whatever you're doing isn't working, and so forth. The guy who currently coaches me is a former #2 at CU Boulder, and he's a great coach and a great player (is from Argentina, worked out with David Nalbandian when they were juniors, etc.)...and he's a monster
(6' 4", 220 pounds, moves like a gymnast).
When we first started last year, I needed some changes to my forehand, volley, and serve. So for a long time, it was just him feeding me balls to get the changes down. Then it became point construction, no score. Now, after a good warmup and check of my current skill level...with corrections as needed... we play sets, where as I improve, he increases his level just enough to stay ahead of me and make me increase my level. After a big point, he'll stop and tell me what I did well or not so well, also on the change games, at the end of a set, and so forth.
It's great training, and I'd encourage everyone, regardless of level, to do the same...with your coach and/or hitting partners. Can my serve, forehand, volleys improve? Of course, just as everyone has stroke issues he or she needs to work on. But at whatever level you currently happen to be, you always need to work on the skills of movement on the court, good tactics, and playing your way through a match point by point, game by game. The opposite, of course, is players who do nothing but play matches and never work on their strokes, strategies, physical conditioning, or mental game. A blend of all of the above works best...and it's also, IMHO, the most fun...