If you don’t understand how hackers could get into your systems, you’re going to have a hard time securing them. Learning how to hack can help implement the strongest possible security practices. Plus, it's fun.
The term “ethical hacker” has received criticism at times from people who say that there is no such thing as an “ethical” hacker. Hacking is hacking, no matter how you look at it, and when one creates “ethical”, one also creates “unethical” and name calling. That said, the work that hackers do for organisations can help improve system security and be quite effective and successful. I use the “usual” pigeon holes because they seem to make sense. They're still just pigeon holes though.