Because Mahdaviat represents what all prophets were sent for - namely, building a monotheistic world on the basis of justice using all the capacities that Allah the Exalted has bestowed on humanity. This is the era that will come after the re-appearance of Imam Mahdi (God's greetings be upon him and may God hasten his re-appearance): the era in which monotheism and genuine spirituality will be dominant over the entire life of human beings, the era in which justice will be established in the real sense of the word.
Saboda kuwa abin da akidar Mahdi take bishara da shi, shi ne dai abin da aka turo dukkanin annabawa da manzanni da shi wanda shi ne samar da wata duniya ta tauhidi (kadaita Ubangiji) sannan kuma wacce aka gina ta bisa asasi na adalci da kuma amfanuwa da dukkanin dama da Allah Madaukakin Sarki Ya arzurta dan'adam da shi. Shi ne dai irin wannan yanayi na lokacin bayyanar Imam Mahdi (amincin allah ya tabbata a gare shi sannan kuma allah ya gaggauta bayyanarsa).