It is maybe an unusual question, but the answer may not be evident if you decided to ask it. It depends not only on the person but also on the environment, social circle, financial situation, and other factors.

Growth Needs

The most common need that people try to satisfy with a hobby is growth need. According to Maslow's hierarchy of needs, it is one of self-actualization (or self-fulfillment) needs.

The main point of these needs is to have the possibility to grow and achieve a person's full potential. When a person can not fulfill this need as a part of professions or family life, a hobby can be the next best choice. It may play a role of a substitution goal that a person creates to have further growth motivation.

It is questionable whether this need and motivation are good for personal growth in the long-term perspective. It is up to the person to define priorities in his/her life. Many will agree that a hobby is an excellent way to spend some time as long as it does not affect more critical areas.

Diversity

Another two reasons why people may have hobbies are boredom and need to look different.

People often become bored because of their life routine. Home-office-home non-stop five days a week. A hobby may introduce a nice diversity in this process and possibly do something that a person may like.

Need to look different and feel different as a consequence is one of a hobby's social aspects. A person may share his/her work with colleagues or friends, get familiar with new people, or socialize somehow. One way or the other, this activity also brings diversity and maybe even progress into a person's life.

Unlike the growth needs, both these cases already involve an element of pleasure, so people get extra motivation to continue taking up a hobby.

Source Of Pleasure

The last common reason why people have hobbies is a desire to have an additional source of pleasure.

People who consider hobby as only pleasure in most cases already have satisfied previously discussed needs, and the hobby became just an entertainment. These people can spend a significant amount of time and money on their hobbies to get as much as they can.

A hobby that is only a source of pleasure can become less interested or even boring with time. It is not uncommon to see that people may change such a hobby for something else. Consequently, people are less involved in such hobbies and rarely achieve the expected level in the appropriate area.

These are the three main reasons why people may have hobbies. The next time you get familiar with a person who has a hobby, you may realize why this person is doing that and understand this person a bit better.