Dust Covers are especially molded plastic or wrapper covers to protect the CPC from dust when switched off. Some people think they have been the most useless hardware add-on ever produced for CPC computers. These accessories have been advertised to "protect" the CPC's dust-resistant sealed membrane keyboard, or even its vacuum-based monitor against dust.
Although being completely useless, dust covers have been (in germany, at least) almost more popular than joysticks. Whereas, it had to be a real dust cover - one needed to buy it (using old newspapers or worn T-shirts would have been déclassé).
Today, Dust Covers are more or less unknown. But, they have left some scars: Many people still feel irrationally guilty when exposing computer hardware to dust. The dust cover phenomenon (eighties) can be compared with the hype for equally useless mouse pads (nineties), protective foils for LCD screens (first decade of 21st century), and sliced bread (thirties).
On a more serious note, mouse pads were a boon prior to all mouses being optical because ball mouses do tend to be a PITA on polished surfaces. (Nowadays, mats are used to protect the polished surface from scratches by them); and dust covers not hindering air flow are on the wish list of any home-server admin, especially those who have experienced dust fires in long-running servers.
In the 1980, many computers were not put in living rooms, but (childrens') bedrooms, attics, basements or even workshops to control wood- or metal-working hardware. Those places tended to be prone to much dust, and the computer was only on for a few hours a day. Consumer products quite often were prone to mechanical jams due to dust, so especially in professional workshops, covers were regularly used. Industry DP was often so prohibitively expensive that home computers - quite often the C64, but the CPC offered a Z80 and CP/M as a boon - popped up in the most curious places (eg. a C64 was used to control the water supply of a German commune in the mid-80s).
Of course, dish towels were used; however, in 1987 a CPC 6128 with GT65 cost DM1.500,-, not counting the Schneider RS232 Interface often necessary for controlling applications, plus application-specific sensors and actors - on 2% inflation for 30 years and in €uro, that would be 1500*1,02^30 / 1,95583 = ~€1.390,-. About DM20,- for a molded dust cover that did cover the machine better really did not weigh that much considering those dimensions.
There are no known laws against producing, distributing, or even using dust covers. However, users are likely to lose their precious warranty if a computer catches fire because of covering its ventilation holes after forgetting to switch off the power-supply. Fortunately, the dust cover users never realized that dilemma, otherwise they'd have probably died from a heart-attack when trying to find a solultion to the question: Risk the warranty, or risk the dust?
As users were often technically savvy, however, that dilemma did occur quite seldom (probably only in metal workshops where the metal-working machine was controlled by CPC when producing metal dust), as the computers were covered only when switched off.