The people of Ancient Egypt believed that the human spirit was permanently linked to the body, so in order to achieve life after death, the body had to be preserved. Without mummification, a person's body would rot, which would destroy the soul.
The people of ancient Egypt lived very short lives. In fact, the average lifespan was only around 40 years, so preparing for the afterlife was extremely important to the culture at the time. All pharaohs and members of the upper and noble classes were mummified. Those individuals in the middle class that could afford the process were also mummified.