For me, the two most important aspects in tennis are keeping it simple (I LOVE SIMPLE) and even more importantly, staying loose. Only following his defeat to Fernando Verdasco in Miami, Rafa Nadal said "it's not a question of tennis. The thing is the question to be relaxed enough to play well on court."
The benefits of staying loose include increased racket head speed and therefore more power, more control, consistency and accuracy, less effort and energy used, less strain on the body and therefore less risk of injuries, a better feeling shot and a better looking shot (which let’s be honest, we all care about a bit!). But on a serious note, if a shot is looking good but not going in, you know you’re on the right path to success and with more good practice the desired results are not far away. In short, being loose will result in a shot that will feel better, look better and go better. So you could say it ticks all the boxes.
Not only do I think it's incredibly important, I feel it is something often over looked by many coaches.
Technically on the groundstrokes and serve, the goal is to create as much power as possible with the least amount of effort. Roger Federer is the best example of this. He is incredibly loose and therefore hits his shots with immense power very efficiently.