Reasons for an SSP administrator to do a full crawl include:
One or more QFE or service pack was installed on servers in the farm.
An SSP administrator added a new managed property.
To re-index ASPX pages on Windows SharePoint Services 3.0 or Office SharePoint Server 2007 sites.
The crawler cannot discover when ASPX pages on Windows SharePoint Services 3.0 or Office SharePoint Server 2007 sites have changed. Because of this, incremental crawls do not re-index views or home pages when individual list items are deleted. We recommend that you periodically do full crawls of sites that contain ASPX files to ensure that these pages are re-indexed.
To resolve consecutive incremental crawl failures. In rare cases, if an incremental crawl fails one hundred consecutive times at any level in a repository, the index server removes the affected content from the index.
One or more crawl rules have been added or modified.
To repair a corrupted index.
The system does a full crawl even when an incremental crawl is requested under the following circumstances:
An SSP administrator stopped the previous crawl.
A content database was restored.
A full crawl of the site has never been done.
To repair a corrupted index. Depending upon the severity of the corruption, the system might attempt to perform a full crawl if corruption is detected in the index.