Apple Support, writing about a very specific software update:
This update is for 20-inch and 24-inch aluminum iMac computers with 2.0, 2.4, or 2.8 GHz processors running Mac OS X Tiger.
It improves the performance and reliability of graphics-intensive games and applications and fixes an issue that some customers encountered when installing Mac OS X Leopard after applying iMac Software Update 1.2.