You never know what you are going to get when you take something from a box of chocolates. I can guarantee you that sometimes you will get one that you like, and other times, well, you know. When I was younger, my mom used to pick up a chocolate and poke through the bottom of it to see what was inside. If she didn’t like it, she would put it back in the box for someone else (read: the kids) to eat.
Managing security on the network is very similar. There are many different types of attacks and it is important to have security solutions that can manage as many as possible. Some of the attacks are easy to identify and mitigate, while others are less appetizing to deal with. Different attack types need to be detected and mitigated. Like my mom, some attacks are better suited to be identified in one location and mitigated (eaten) in another.