Simply, they are to give mobility to people who cannot walk fully due to disability, injury or illness. Whether you need it briefly when recovering from an injury, or as a full-time addition to your life, wheelchairs are designed to help you get around the world!
Self-propelled wheelchairs are usually designed in mind of the user needing to manoeuvre their way around the world. Useful features such as a half folding back are perfect for this, enabling people to reach for objects behind them with greater ease.
Attendant wheelchairs are more often used for people with temporarily restricted mobility, suffering an illness or injury, or for users who are incapable of moving themselves in the chair. These are designed with lightweight mobility in mind, easing movement for the attendant.