I don't know what it's exactly for since I'm not an expert in harddrive settings.
It has something to do with Raid settings in bios, and the OS will not recognize single harddrive if you don't install the driver before OS installation.
You will see an option to install drivers in Vista installation screen.
If you go into the bios/hard drive settings (can't remember what it was called exactly), you'll see 3 settings - Autodetect ATA ,Raid, and Autodetect AHCI(or ACHI, can't remember, it was done at like 3AM).
Factory default on this option is AXXI (3rd option), and if you don't install the driver, you will get stuck at the end of OS installation.
You can also change the bios setting to the first one - Autodetect ATA, but you will have to disable FCM(intel turbo memory) first. With this option chosen, you don't have to install the driver.