It's like breaking into a house to expose security flaws.
When you are a 'white hat' hacker you use hacking to find weaknesses in the system. If one person can break in then someone else can, but if you can figure out what flaws there are you can fix or 'patch' them. 'black hat' hackers are the reason companies hire 'white hats' to find vulnerabilities, because 'black hats' hack systems maliciously. For example breaking into the USA Pentagon system to try to hijack it.
Bottom line: If you have a really good hacker try hacking your system to find security vulnerabilities, it is similar to you hiring a really good burglar to see if your house has any security issues.
Hope this answer helps you understand what you needed too.
May the Force Be With You while you do your homework!
-Jabba
