SQL Server Restore from one database to another
"It just does".  Its just meta data about the backup.

The Books Online article http://msdn.microsoft.com/en-us/library/ms186299.aspx hints at it, albeit not very well in the "Remarks" section.  From that article you'll see a reference to sp_delete_backuphistory.  So yes you can safely remove it if you need to.

The info that goes into that backupset table, from a restore, is pulled out of the backup itself.  Run:

RESTORE HEADERONLY FROM DISK = 'Path to backup file';


and you'll find the information.