It is a fact that Allah did not have any Personal interest in the creation of the Universe because He is above all considerations of wants and wishes. He has created the world with all its wonders to be exploited by His noblest of creatures, man!
It is therefore natural that the human beings, to achieve excellence, needed a mentor, endowed with all the superior qualities by none other than the Creator. The mentor had the duty of bringing out the human beings from the darkness of ignorance to the light of knowledge through the Guidance received by way of Revelations. It was also necessary for the mentor and guide to be one of the humankind that his compatriots feel comfortable with him and at the same time he had to be endowed with Spiritual Excellence received through Revelations from Allah.
It is recorded that a fire-worshipper came to Imam Ja’far As-Sadiq (as) and asked him some questions. One of his questions was that why there was a need for Allah to send a Prophet, a Messenger?
The Imam (as) replied to him thus: “I have already explained to you about the Existence of Almighty Allah. He is the Creator of everything and all the living creatures in the universe. Everything happens in the universe with His wish. His creations cannot see him with their eyes. Their senses cannot gauge Him. He has no body that one could go to His physical presence and converse with Him.
Therefore, it was imperative that there should be a medium through which His Guidance could be communicated to the human race for treading the right path. Therefore Allah’s selected few, who were termed nabis, prophets or messengers, were the possessors of Allah’s most desired characters and were of the same race and physical features as their flocks. These messengers have been endowed with certain maujezas, or incredible miracles like bringing back the dead to life, restoring eye-sight to the blind, curing incurable ailments, breaking the moon into two, bringing back the sun which had already set etc, etc. The world has never been devoid of such nabis. If at a time there is no nabi, then there will be his vicegerents to carry on the work of the Mentor.
Man is a gregarious being. Every individual is dependent on the others for his needs. This need sometimes becomes the cause of conflict, differences and even wars. Therefore he needs a ruler who acts as a Hakm, or arbiter, whose verdicts are fair and above all personal considerations and is infallible.Such a Hakm can only be identified and put in place by none other than the Great Creator, Allah.
Therefore it is cardinal that the Nabi and the Imams are mansus-min-Allah or selected and appointed by Almighty Allah.

Mu’jizah Or Miracle of the Prophets
As a proof of Prophethood, the Prophets’ present miracles to the people. These miracles are incredible happenings that the commoners are unable to perform. For example: converting the staff into a snake, making the dead alive, breaking the moon into two etc… etc…
When a person claims to be the messenger of God and performs miracles as a proof, then there will not remain any doubt as to his veracity. To the contrary, when an imposter claims that he is a prophet, then it is impossible that Allah would provide him the means of performing any miracle. As people believe in the veracity of a Prophet by witnessing the miracles, so also people believe in him by hearing about the miracles through authentic sources. It is just like people who have not seen a city, say for example Mecca, believe in its existence only on hearing about it from others.