The Advent of Divine Elects
The main reason for the arrival of new non-Law-bearing prophets in the Muhammadan Ummah (pbuh) is that Islam came for the whole world and must be propagated. And for this important work two arrangements are necessary: first the internal reform among Muslims (i.e., the strengthening of their faith in Islam), and second, the spread of Islam around the world. These two needs cannot be achieved without strict discipline, a leader, an Imam.
“Allah would not leave the believers in that (state) you are in (presently) until He separates the evil from the good. Nor would Allah reveal to you the unseen. But (instead), Allah chooses of His messengers whom He wills, so believe in Allah and His messengers. And if you believe and fear Him, then for you is a great reward.” (3:180).
Here it is said to the believers: Oh you who claim to have faith, as long as there shall be no difference between the good and the bad you will remain mixed up with the result that Islam will be weakened. Allah does not like such a situation. Surely He will send a prophet to separate the bad from the good. Since only God knows who is good and who is not, and He alone knows how to separate, He will choose a pious Muslim to operate that separation of/ distinction between (the good and evil/ bad ones). O you who believe! “So believe in Allah and His messengers.”
On the other hand, if there should not come a prophet, why did Allah (swt) say to those who believe in Hazrat Muhammad (pbuh) that a time will come when there will be no difference between good and evil and that He shall then send a prophet? Why did He give the command to believe in the Prophet and to fear (i.e., to reform)? There is in all this food for thought.