If I understand your question, you are asking why someone would start a catering business.  I have edited your question to say this.

The most important reasons for starting a catering business are these:

• Perceived demand.  If a person is going to start a catering business, they have to think about whether there is demand for that particular service in their community.  You would not want to start a catering service if you did not think there was enough demand (if there were too many caterers already or if people in your area were too poor to afford caterin, for example).
• Expertise and interest of the entrepreneur.  In order to start a catering business, you would need to know what you are doing and you would need to enjoy that line of business.  So one of the major reasons to start a catering business would be personal--because you are good at it and you like it.

So there are two main reasons--you would start a catering business because you think you would be good at it and because you think you could make money.