Why did economic specialization become
common in civilizations?

Job specialization allowed for people to develop and contribute important skills to help maintain their civilization. An individual could not develop all the skills needed to maintain specific items and tasks. Most people in early civilizations were farmers.

cultural/diplomatic relations. most medieval japanese   culture has chinese origins. writing, architecture etc.

islamic religious leaders have traditionally been people who, as part of the clerisy, mosque, or government, performed a prominent role within their community or nation. however, in the modern contexts of muslims minorities in non-muslim countries as well as secularised muslim states like turkey, indonesia and bangladesh, religious leadership may take a variety of non-formal shapes.

compared to religious leaders/clerics of the other abrahamic faiths, islamic clergy are said to resemble rabbis and not priests. unlike catholic priests they do not "serve as intermediaries between mankind and god",[1] have "process of ordination",[2] or "sacramental functions",[1] but instead serve as "exemplars, teachers, judges, and community leaders," providing religious rules to the pious on "even the most minor and private" matters.[1]