Why do people string the racket at different tensions?
The goal is to string the racket in a way that allows you to optimize your swing speed. The typical tension in the winter is string the racket much looser, in the summer string the racket tight. the heat affects the balls and they seem lighter in summer, so they need to control the ball by tighter strings and normally people string the racket three times a year. This to account for the weather and strings going dead.