From Discworld & Terry Pratchett Wiki
Imps may be considered small demons without much demonical power. Imps are widely utilized as painters in iconographs, or memorizers in Dis-Organisers. Some time ago, imps were also used to pedal, as a power source, to run a watch (portable timekeeping device), but such watches were huge (in order to house the imps), kept lousy time, and after a while the imps would evaporate and disappear. Imps who live and work in iconographs and Dis-Organisers seem to last longer, if they are regularly fed. Imp-painting iconographs paint very accurate pictures almost instantaneously; imps have no imagination whatsoever, so the painting is absolutely accurate, and very useful in law-enforcement (see Feet of Clay and The Fifth Elephant). Imp-memorizing Dis-Organisers, on the other hand... well... they are called Dis-Organisers for good reasons (see Feet of Clay). However, sometimes the imps in Dis-Organisers seem to be able to see through the space-time continuum and, for example, tell the owner of something that is about to happen (see Jingo). Some imps in the more advanced makes of Dis-Organisers can perfectly memorize long conversations (see The Truth). Very small imps called nano-imps are used in iconographs which spying diplomats carry in their pockets (The Fifth Elephant). These nano-imp-iconographs and advanced Dis-Organisers can cost from a hundred to several hundred Ankh-Morpork dollars.