String tensions: The looser the string usually more power from the ‘trampoline effect’ of the contact surface.
The tighter the string, the more control, which allows beginning players to work on their swing speeds and hopefully still keep the ball in. A more advanced player uses tighter strings for finesse and when they desire power they really swing hard, the best of both worlds. However, comfort is also a must now a days, right?!
Looser strings allow the ball to comfortably come off the racket and strings, sometimes too much, but their is an easiness to the contact of the ball. This is good for beginners who start out with a slower string. Advanced players and pros like this ‘easiness’ but they can also can swing very fast if they desire, but they use spin to counter all the power etc.