This famer once acted as a financial partner to a man who started a business sawing wood. This man once suggested that he could cut down his own logs. My friend told him:
While I am your partner, you will do that over my dead body!
The fact is the man's idea was silly. If he were to cut down his own logs, during that time he could be sawing them. He could never cut down logs with the same efficency as a person who was logging for a living. Plus he would have to invest more time and money on transport.
Division of labour is essential to economic efficency. A man who practices every trade will be a master of none of them.