Bad-Guy Tools for Good Guys
Script kiddies, Far-Eastern organized crime gangs, disgruntled employees: you name them and the chances are that at some point in the future your network will come under attack from them. In fact it probably already has.
Network security is a tricky problem for administrators because there's nothing you can do to make your network completely secure forever--security is a continuous process, and the aim must be to make it secure enough to make the risk of penetration acceptable, at an acceptable cost.
One of the difficulties is that in order to decide if your network is secure enough, you need to know how secure it really is. One of the best ways to see how secure your network really is, is to see how easy it is to break in to it and compromise machines connected to it.
There are many, many security Linux distros out there, and it would be naïve to assume that they are all really aimed at security professionals wanting to check their networks.