Got this wrong but... found in BOL 2005 the following remarks.
Updates information about the distribution of key values for one or more statistics groups (collections) in the specified table or indexed view.
Usually an index is being updated if there is one as well as other information about the table. I do agree that it is assisting the query optimizer. So the answer is a bit shallow because in reality if you run say sp_updatestats and an index needs updating you will see that occur. The end result is the the query optimizer can use this information to perform best.
Chad E. Downey, CDMP - Certified Data Management Professional
Consultant - SQL Server, SSIS, SSRS, SSAS