Killer right here for an increasing percentage of the population. Fewer households even HAVE a desktop PC these days, laptops, tablets and smartphones are the computing devices of choice for many these days.
Even if you HAVE a desktop, there's no reason to assume the power supply can handle a dedicated graphics card if it wasn't shipped with one. Many OEM's like to skimp on power delivery at the price points the majority of consumer desktop PC's are sold at, since a $400 machine has little profit margin in the first place.