The Windows registry is in essence a database of settings for whatever software wants to use the registry, presumably making it more efficient and easier for programs to use consistent settings when working together. There are a number of reasons why a registry can become broken / corrupt: viruses, deleting the wrong files, a software installation that goes wrong. A corrupted registry can lead to warnings, crashes, things not working as they should etc, so should be fixed whenever an issue arises, and for good housekeeping should be scanned (and issues actioned) on a regular basis, just as one would a standard virus scan.
