First of all, spending time alone is the better way to refresh people than spending time with people. People usually feel more comfortable when they are being alone than being with people because nobody disturbs them so that they do not need to care about other people; and that enables people to recharge themselves easily. For instance, in the case of students or workers, they spend time at school or work places for the most of day, and they are bound to be tired due to studying or working. In that a quality of work can be changed which depends on how well use their own free time, spending alone with feeling comfortable and no disturbance is necessary for the next day.
On top of that, having my own time is helpful for self-development. If people spend time with the others, it is mostly easy to spend a meaningless time such as chatting with friends at the café, walking around by doing window-shopping and watching a movie. However, spending alone enables people to have their own time for self-development such as registering an academy what they want to take, having their own time to study. Also, people can have a time for hobbies in which they are interested.
Lastly, if we look at this from the side of saving money and time, spending time alone also is a better way for them. To spend time with people, it is unable to avoid wasting time and money because people would have to use public