The main constitutional function of the Deputy President in a presidential system such as Kenya's (and the United States') is to secure an efficient and least disruptive transition, whenever a vacancy arises in the President's Office.
While the Deputy President is constitutionally mandated to serve as the principal assistant to the President, there is no doubt that the President does not suffer for want of assistants in the discharge of his constitutional mandate. The entire Cabinet is at his beck and call, while the bureaucracy stands at attention to his every directive.
Consequently, the Constitution in Article 146(2)(a) recognises that the Deputy President serves as President for the remainder of the President's term should a vacancy arise in the latter office.
If the unexpired term of the President served by the Deputy President is in excess of two and a half years, the constitution deems the deputy president to have served a full term for purposes of Article 142(2).
That means, for instance, a Deputy President who hypothetically serves an unexpired term of three years commencing mid 2019, will be eligible to contest the 2022 elections but not the 2027 election, as he would have served two terms and is therefore barred by dint of article 142(2) of the Constitution. Such a deputy president will therefore serve a total of only 8 years if he were to be elected president in 2022.