In order to keep yourself secure you must understand your enemy.
Prevention is the only protection from becoming the victim of a security exploit.
The first step in doing this is to determine what services your servers offer, so you can secure them in the best manner possible. Network scanning can be used to determine potential communication channels. Mapping their existence facilitates the exchange of information with the host, and thus is quite useful for anyone wishing to explore their networked environment, including attackers. Scanning, as a method for discovering exploitable communication channels, has been around for ages. The idea is to probe as many listeners as possible, and keep track of the ones that are receptive or useful.