The most obvious reason is if the standard installation procedure produced problems; an Erase and Install, followed by a restoration of your data from backup, could solve those problems. You might also want to consider this approach if your Mac has been acting buggy under Leopard and you suspect you may have some hard-drive or directory corruption; erasing the drive before installing Snow Leopard could fix such problems.
After installing Snow Leopard this way, you’ll be asked if you want to transfer applications and data from your backup. If you choose to restore everything, you’ll end up essentially where you would have been if you’d used the standard install procedure. (Under Leopard and earlier installers, using Erase and Install and then restoring your data using Migration Assistant would often fail to transfer over all your data; specifically, some settings and add-ons would be left behind.)
The $29 retail Snow Leopard upgrade disc—as well as the $10 upgrade disc available to people who purchased a Mac on or after June 8, 2009—is for users who already have a license for Leopard (OS X 10.5). Apple says those with Intel Macs running Tiger (OS X 10.4) must buy the $169 Mac Box Set, which includes a full version of Snow Leopard bundled with the latest versions of iLife and iWork.
However, the Snow Leopard upgrade DVD does not require that you have Leopard installed; it’s a full install disc that works on any Intel Mac. Similarly, as with every version of Mac OS X, Snow Leopard does not require activation or a registration code; Apple continues to rely on the honor system. This is great for Leopard users, as it makes it much easier to install Snow Leopard on your Mac without having to jump through hoops; for example, if you decide to erase your hard drive before installing Snow Leopard, you don’t have to first install Leopard.
Snow Leopard appears to be the easiest, and smartest, Mac OS X install yet. Just be sure to perform a full backup beforehand; as long as you have that, you should be set.