Unless there is a very special pillow design, all models come together with their adjacent pillow covers; these external items are the ones that actually achieve the artistic effect in the case of decorative items. We should make a clear distinction between the pillow covers used to sleep on and the pillow covers used for the nice cushions on armchairs or sofas. The former are only made of certain types of material that are good for direct skin contact: cotton or silk on a regular basis. No synthetic texture should be used for the pillow covers you sleep on, as they increase the risk of skin infections and irritations.
Sleep pillow covers are usually bought in matching sets with the rest of the sheets and mattress covers; they have the same material, the same design and look great in any bedroom. Pillow covers or casings should be changed at least every weak, or even more often in case you have a skin problem such as acne. They are easy to wash and likely to last for a large number of uses. Not the same thing could be said about decorative pillow covers that usually require extra care when it comes to cleaning; due to the artistic pattern used in the design, such pillow covers are more exposed to deterioration.
We should not forget the fact that most pillow covers need to be a perfect match for the rest of the room decorations: too adorned items would contrast with a very simple interior design for instance. Sometimes, there are special cases when decorative pillows have no casings at all; this is the situation with leather made cushions that are the match for leather-covered couches. Nevertheless, such examples are pretty rare, as in general house owners have regular pillow covers, which can be alternated according to the season for increased good looks.
Thus you can very well use some great pillow covers with Christmas or Easter decorations; you can also alternate pillow covers with flowery or fall motifs depending on the season. The flexibility of use specific to such elements allows one to add an ever new colorful look to any house corner. Pillow covers can be changed according to the house owner's wish particularly since their main role is to give comfort and warmth. Sometimes, old drapes provide the material for pillow covers, and all one needs to do is to be skilled enough to design and sew the casings at home.