What is the importance of having a written law code

A written law code (as opposed to a unwritten one) provides all sides a security that the law will stay the same and that all people will have a fair access to it - for example if the law says something about a marriage, you can be sure that this won't be twisted later by people interpreting the law differently (as could be the case in an unwritten law) - in case of doubt, you can look at the original

so there woundn't be any conflicts of the sort of "I understood the law to be this" "I understood it differently" - if the law is written down, it can be always accessed in original by all interested parties.