Umar b. al-Khattab asked Ka'b who the lords of knowledge were. He replied that they are those who act according to what they know. The goal of Muslim knowledge is to seek the pleasure of Allah and to create and maintain an environment in which one can act without hindrance for the pleasure of Allah, worship Allah as He should be worshipped, and follow the commands of Allah and avoid the things He has prohibited. It is exceedingly difficult to put one's knowledge into practice if one is drowning in a sea of usury, bereft of any leadership to show one which way to swim, let alone how to get out. Hence the neccessity of recognising the destructive and haram nature of usury and the fact that amirate is necessary to help implement action by knowledge.
Action is necessarily wider than the sphere of the individual.
It is also important not to turn Islam into a private religion and divorce it from the arena of political and social action - as the Reformation ultimately did to Christianity by demolishing the political unity of Christendom and replacing it with the nebulous area of private conscience. This results in complacency and a gradual weakening of Islam, an erosion of its natural economic, political and social thrust. What was once a small crack soon becomes a vast chasm. The healing involved is of a surgical nature. It is cutting away the source of the poison to heal the body.