A clean install is not always necessary. Sometimes, users create their own settings in the Nvidia Control Panel, rather than keep everything at default. There are even settings specific to each individual game that can be tweaked even further. If you are someone with a lot of individualized settings, you probably don't want to do a clean install and lose all those custom settings. If you don't use a lot of custom Nvidia Control Panel settings, then you probably do want to click the clean install option.
A clean install is supposed to be like starting fresh, but the Nvidia installer is much improved these days and the clean install option is not necessary for a hassle-free installation. Generally, for someone like me who does have...