Pillow cases were invented in order to keep pillow clean and prevent them from having a smell. At first, pillows were made of pure cotton, which is known for its properties of liquid absorption and smells faster. Over long periods of time, and due to the continuous absorption of sweat and other liquids; the pillow will start to smell. Pillowcases are also made of cotton, and they do absorb sweat; but they are easier to clean and faster to change. Even after the invention of fabrics and their usage in making pillows, pillowcases are still being used worldwide for the same purpose.
Pillow cases have enormous number of designs, because they can come along with a set or be sold separately. If they come in a set, they will have the same design as its set of course. The separate pillowcases have many designs and colors, they can come in solid colors or with paintings and drawings. There are pillowcases that can match other sets, which is great in case you need to replace the pillowcases alone without changing the whole set.
There are many standards for pillow cases sizes, and they are all according to the set and the bed they will apply to. There are many sizes that fit every bed size, the king size, queen size and twin. Each bed size has its standard pillow size, which is the main measurement of the pillowcase size. The main material for pillowcases is cotton, for its ability of absorbing sweat and liquids. But recently there have been some other additions to the pool of materials, like fur; which is used in colder areas for more warmth.
Every pillow needs pillow cases, even if it was not on a bed. Some pillows are used on sofas and chairs, these also need pillowcases to prevent them from being stained or wet. The variety you find in the design and the material of pillowcases are great, it will give you the chance to find exactly what you are looking for. There are also many online catalogues for famous pillowcases resellers. You can go through this as well.