A vulnerability scan is one a common tactic used by a hacker to gain intelligence about potential targets on a network, whether these be PCs, routers, servers or websites.
A good comprehensive scan of an internal network, followed by some initial investigation and summarised in a report can provide invaluable input for an IT administrator or support provider to help harden your network.
The reporting in and of itself is also useful documentation to keep for compliance purposes, demonstrating as it does that your organisation has taken a proactive approach to security.
Basic External Vulnerability Scan
The most basic scan is performed from outside your network on a regular basis, for example quarterly, to scan for any weaknesses or vulnerabilities that can be seen publicly (via the Internet).
Internal Vulnerability Scan
This is an excellent way of finding vulnerability that a hacker could exploit, after they have breached the perimeter.