Before the creation of the 12 Tables why did the Roman plebeians want a written law code?

Usually ancient civilisations did not have a written law at the beginning. The origin of their laws lied in religious rules and ritual procedures. Religious, private and published laws were undifferentiated and knowledge of rules were held by priests, who held it in secrecy. This easily led to abuse and manipulation by the rich who had connections with or were priest. This situation could lead to popular demands for written and published laws so that the rules could be known more widely, giving more transparency and fairness and more of a protection against misuse or abuse of power. Written legislation also gave a chance for legislative reform.

This was a usual pattern, but it does not mean that this process happened in all civilisations.