Any network connected to the Internet can expect to see malicious traffic. Now, this could range anywhere from something like compromise of your system or an intruder connected to your network, or it could be something as simple as browsing the wrong website that attempts to use the latest Flash or Java exploit.
If your network is hosting anything of value, it may make sense for you to monitor this sort of traffic. It will allow you to notice the laptop infected with Malware that is probing your other systems.
Alternatively, maybe you just want to watch your network traffic in order to detect misconfigurations. Perhaps one of your systems is misconfigured, resulting in it hammering away at your server. IDS systems can be flexible enough to catch any sort of traffic that you would like to look for.
