Though you may know and follow basic security measures on your own when installing and managing your network and websites, you'll never be able to keep up with and catch all the vulnerabilities by yourself.
Vulnerability scanners can help you automate security auditing and can play a crucial part in your IT security. They can scan your network and websites for up to thousands of different security risks, producing a prioritized list of those you should patch, describe the vulnerabilities, and give steps on how to remediate them. Some can even automate the patching process.
Similar to packet sniffing,port scanning and other "security tools", vulnerability scanning can help you to secure your own network or it can be used by the hackers to identify weaknesses in your system to mount an attack against. The idea is for you to use these tools to identify and fix these weaknesses before the hackers use them against you.
The goal of running a vulnerability scanner is to identify devices on your network that are open to known vulnerabilities.