Archived - Mac OS X: About the "Install Mac OS 9 Drivers" Option in Disk Utility
If you do not select this option when formatting a hard drive, the disk may not appear when the computer starts up from any version of the Mac OS other than Mac OS X. You should only deselect this option if the computer or hard disk is to be used exclusively with Mac OS X. Not selecting this option saves a small amount of drive space.
This option does not affect the availability or use of a disk with Classic in Mac OS X.