I'll try my best to answer your question - I don't quite understand what you're asking when you say
punches their racquet's strings and changes the racquet after that
If you are a beginning tennis player, the rule of thumb is that you should restring your racquet as many times in a year as you play in a week. So if you play twice a week, you should get your racquet restrung every 6 months. Some people play with the same strings for years or until the strings break.
The reason some players get the strings replaced before they break is because of tension loss. It depends on the string material used - but all strings lose tension after being used for a while.
For professional players, they get their racquets restrung constantly because any amount of tension loss can potentially affect their shots, so they try to minimize how much that can happen. A single player could get anywhere from 2-10 racquets restrung for a tournament. The very top players (like Federer, Nadal, Djokovic, Murray, etc.) can get about 8 racquets restrung for them before every match they play. They pay a professional racquet stringing company to travel with them to various tournaments to restring their racquets for them.
Unless you are a fairly advanced player, you are not likely to notice a little difference in the amount of tension in strings - like 3-5 kg difference. But for advanced players, they will notice because they have a better feel for the tension.
Tension typically affects the amount of power (or control) you get from the strings. The lower the tension - the more power you get. The higher the tension, the less power (thus control) you get. It's just like a trampoline - a loose trampoline will bounce you higher in the air than a tight one will.