Sole Heir

What happens if these is only one heir?

Then the instructions of the Inheritance Act of 1965 regarding the share/s in the estate will apply to all the estate , and those regarding the division of the estate will apply to the merger of estate property with the property of the sole heir. 

Is there any point in having an executor for the estate if there is only one heir ?

In principle there is nothing to prevent this, if there is a need, even if there is only one heir. For example, if the deceased made a will in which he expressed his wishes for an executor to be appointed for the estate, and even gave instructions, or where the heir him/herself asks for this, then there is no reason not to appoint one.