Firstly if you know how to cook you can make healthy meals instead of just going to Macca's or Hungry Jacks all the time.
I mean its true that some fast food places do serve healthy food. Some examples are Oporto, Subway and other but most of those places are expensive and don't get a lot of customers cos of competition from Hungry Jack's
Cooking is a skill that everyone should know. People should know how to make their own food. People who don't know how to cook are more inclined to eat out and eat more unhealthy food. Though, yes, not all food in restaurants are unhealthy, most are. When you know how to cook, things are more convenient and you food is healthier. Also, cooking food is less expensive than eating out.
Food ordered from restaurants does not look as healthy as it is. Home-made foods are more powerful than ready-made foods. Home-made foods are healthy because we can adjust the sugar-level or oil-level we expect to be consume by ourselves. Ready-made foods are unhygienic. Give him some food and he will live for a day, teach him how to cook and he will live for a lifetime.
Yes, it is important to know how to cook, because cooking is a basic skill. Most people will be in a position in some point in their life that they need to know how to cook. At some point, they will need a specialized diet and they will need to know how to make those foods for themselves.
It is important to know how to feed yourself in a healthy way, but that does not mean you need to cook. Cooking is really not that nessesary in society today as you can get all the ready made meals. Cooking is just really not nessesary with so much food available.