Protectors are also there as an allergen and dust mite barrier and, of course, to protect against dust, perspiration, body oils and even fake sun-tan so they are really worth considering. There are different types of protector, as explained below;

Quilted protectors are soft to the touch, and suitable for those who simply want a layer between their mattress and the sheet. They are very comfortable, however, there is no waterproof element to these.  

Side fitted waterproof protectors are the most popular and look like a fitted sheet and tuck in a little under your mattress. These will protect the top of your mattress from spillage and accidents.  The sides of these protectors are not waterproof.  These are easy to remove for washing when accidents occur.  They do not need washing every five minutes as they are designed to fit under a fitted sheet.

Side Fitted Waterproof Protectors/Sheets - these are the same as above, however, can be used as a dual purpose protector and sheet.  Obviously if you use this type of protector as a sheet you will need more than one. 

It is important to note that stained mattresses can affect your warranty.