MD enjoys the highest position in the BOD. He is entrusted with all the major responsibilities for the smooth running of the business. Hence the law wants the shareholders to watch and analyse the performance of the MD for the four years and then to take a judicious decision whether to reappoint him again or not. The provisions of sec 196 protects the shareholders from taking any hasty decision regarding reappointment. In other words even the Md is prohibited to manipulate with the decisions of shareholders by showing good performance in the initial years and securing a reappointment for himself well before the end of his tenure.
Disclaimer: This explanation is my own logical thinking and has not been taken from any secondary source.