They reached consensus on the obligatory nature of the appointment of the Imām, the only difference being as to whether it is obligatory on Allah or on people, with transmitted or intellectual evidence.
The position of the people of the Sunnah and the great majority of the Mu‘tazilah is that it is an obligation on people because of:
1. evidence transmitted in his words, may Allah bless him and grant him peace, in that which Muslim narrated in the hadith of Ibn ‘Umar, may Allah be pleased with him, in this wording: “Whoever dies without an Imām dies the death of the time of ignorance,”
2. and because the Companions, may Allah be pleased with them, considered the most important of all matters to be the appointment of the Imām to such an extent that they gave it precedence over his burial, may Allah bless him and grant him peace,
3. and because there is no avoiding the necessity of the Muslims having an Imām who undertakes to execute their legal judgements, establish their ḥudūd limits, protect their borders, equip their armies, take their zakat (ṣadaqāt), conquer insurgents, thieves and brigands, establish the jumu‘ahs and the Eids, marry off young people who have no guardians, divide up the spoils of battle, and the similar duties of the Shari‘ah which individual members of the Ummah cannot take upon themselves.
[Mulla ‘Alī al-Qārī, منح الروض الأزهـر في شرح الفقـه الأكبـر (Minaḥ al-Rawḍ al-Azhar fi Sharḥ al-Fiqh al-Akbar), Dar al-Basha’ir al-Islamiyyah, Beirut, 1998, p. 410]