Some do it because of retirement. Some do it simply for something to do. Some do it because of a lack of fulfillment from any other aspect of life. Hobbies and recreational activities supply an outlet for creativity, a sensible longing, a source for relationships. Even the solitary activities like gardening or crafts provide an outlet for creativity.
All kinds provide for a sense of accomplishment.
According to Dr. Kirk Miller Ph.D., a licensed psychologist, any hobby or recreational activity is a stress reducer. People do it because they care about it. It gives people reason to get out of bed besides a job or any other obligation. If the activity is physical it is an antidote for the depression which accompanies a sedentary lifestyle. Velvet Adrian LMFT, a licensed marriage and family therapist, says it gets a person busy, keeps the mind engaged, and is a way to form relationships with like-minded people.