According to Imam Khomeini, the Most Noble Messenger (PBUH) headed the executive and administrative institutions of Muslim society.
In addition to conveying the revelation and expounding and interpreting the articles of faith and the ordinances and institutions of Islam, he undertook the implementation of law and the establishment of the ordinances of Islam, thereby, bringing into being the Islamic state.
He did not content himself with the promulgation of law; rather, he implemented it at the same time.
After the Most Noble Messenger (s), his successor had the same duty and function. When the Prophet (PBUH) appointed a successor, it was not only for the purpose of expounding articles of faith and law; it was for the implementation of law and the execution of God’s ordinances.
It was this function—the execution of law and the establishment of Islamic institutions—that made the appointment of a successor such an important matter that the Prophet (PBUH) would have failed to fulfill his mission if he had neglected it.
For after the Prophet (PBUH), the Muslims still needed someone to execute laws and establish the institution of Islam in society, so that they might attain happiness in this world and the hereafter.