I first saw one in Dubai about three years ago and bought it for about a tenner. Portability was the main consideration. I travel extensively with my laptop and spend several hours with clients on their premises working on in-house projects. Using a cramped laptop keyboard without a separate numerical keypad for several hours gets uncomfortable.
I also find it useful when a wireless keyboard and mouse set refuses to respond when batteries are changed and need synchronising which only seems to work when you have Windows loaded. (And you can`t do that easily without a keyboard or mouse) At home I can attach it to the laptop as well and work on the dining room table - no scratches! Easy cleaning - wash it in the sink. Dinners Ready! Roll it up and put it in a drawer.