Leaving the compulsions by law or the like,is IA necessary ,if the business is managed by the owner himself.My opinion is necessary.Am I right?

The audit is the eyes and ears of the owner. So it depends on the size of the organization and the owner's self-awareness.

The bigger or more complex the organization is, the more it needs Internal Audit. 

The less control-aware the owner is, the more he needs Internal Audit. 

So the need for Internal Audit can be mathematically expressed as:

Organization Complexity X1/Owner's Control-Awareness = Need for Internal Audit