For some people, growing their own food is not worth it. This is because producing, say, 10 kg of food for them may require 20 hours of labor. If they had instead applied that 20 hours of labor in something like accounting, engineering, or office administration, they may make enough money to buy 100 kg of food.
However, some people are not able to increase their hours easily. For example, last week at work I asked the supervisor if there was any extra work the next week. She said no. Thus the next week I stay home and do nothing. I may as well use this time to do something useful, such as grow my own potatoes. At the moment I don't need to do this because my parents fully subsidize my food costs. However, if I am at home and I am bored and I wanted exercise, why not use this time and effort to grow my own potatoes and then try to sell these potatoes to a local store?
The reason why I am interested in growing my own potatoes is because I hear that they are easy to grow. Rice and wheat require lots of labor. This is an area in which I am not an expert, so don't quote me on what I have just said. Of course, I won't just eat potatoes all my life. Potatoes will be my core food. I will buy other foods for the sake of diversification across different nutrients and also because of taste and variety.