Why did the plebeians want laws written down?

Originally law were kept secret and the proceedings of the senate were not published. This left a lot of room for abuse of the poor by the aristocracy. The plebeians wanted written and published laws and prosecution procedures so that the rules were known by everyone, restricting abuse and giving people a chance to appeal. The uppermost grievance of the poor was the abuse of defaulting debtors by the (rich)creditors who imprisoned them, tortured them, and sometimes sold them as slaves. The Law of the Twelve Tables also established court procedures which provided a degree of protection for the defaulting debtor. The law also established clear standard penalties for crimes.