You can revamp tired and worn chairs by covering them with new materials, including crochet. You can use any number of yarn types to crochet with, however it is important to select a specific type if the chair will be used a lot and the cover needs to be durable, or if it is a rarely-used accent chair and you want to showcase an intricate pattern.
Wool is made from animal hair, most commonly sheep. It is durable, waterproof, colour-fast, resistant to dirt and wrinkles and wicks away moisture naturally. These are desirable qualities in a yarn if you are covering a chair that is used frequently. It's wise to select a medium or heavyweight grade of wool to crochet a substantial and durable chair cover.
Mercerised cotton yarn is made from the fibres of the cotton plant. It is an ideal yarn to showcase intricate lace patterns, as it does not stretch. However, if you make a mistake when crocheting with cotton, it will be glaringly obvious. Cotton yarn is durable, breathable and absorbent, and will work well for a cover on any chair if you want to dress it with intricately-crocheted lace.