Because it relates to the prior mode, the HD or windows is set up on.
In other words depending on how and what interface mode is used.
If you look in the bios and the mode for the hard drive is set to Sata.
Then you are required before installing windows, where on the main page you are asked what hard drive to install windows to. if you reach that point.
To pre load the sata driver for the interface the drive is selected on in the bios.
If you were provided with a driver cd, then it should be inserted and you should select the sata driver. Once loaded. a drive should be listed to install windows too.
The Sata drivers. if you look there is and option in blue to the bottom left to add or install the driver before the setup of fresh...