Why is pentesting required?

Pentesting or Penetration testing guarantees that you take precautionary safety measures to improve the security of your web hosting applications.

Is PenTesting harmful?

It depends how you use it. Mostly ethical hackers or (white-hat and grey-hat) hackers use it for exploration of the network architecture; find and report bugs to the parent organizations.

Famous Organizations involved in PenTesting?

Google has a security exploration team called Project Red dedicated to this.