Although most current design hard drives are 1 inch high, some large capacity SCSI drives are too tall to fit. The Y adapter is used to add a second drive power connector for the new drive. The purpose of the Y-adapter is to provide an additional power connection for the drive to be added.
All information here is for reference only.
Working inside any computer can be potentially dangerous to you or your computer. If you're not qualified, get someone who is to do this. You assume all risk from any work done inside your computer. Modifications or upgrades may void the Apple warranty. Always make sure power is off and power cables disconnected before working inside any electronic equipment.