Q: Why use Computer Groups in WSUS?
A. Computer Groups in WSUS enable you to target the updates you have approved. By default, there's an All Computers Computer Group you can use. But if you want more granular targeting of updates, create separate Computer Groups containing the machines you wish to specify for different collections of patches.
You can manually assign machines to Computer Groups or configure a machine's Computer Group allocation as part of a Group Policy via the Enable client-side targeting setting.