Why were the 12 tablets created?

Roman law was kept in the memories of the aristocracy, so any lesser person who had a problem had to approach one for information and help. The aristocrat gave the help without charge, but it made the aristocrat the patron of the person, who became his client, and was obliged to support him politically and financially when required. The lower classes, as they began to seek a degree of independence, demanded that the laws be written so that they could understand what their legal obligations were in advance of getting into problems, and so have a chance to avoid them, rather than just finding they were in legal trouble after the event.

As a result the aristocrats grudgingly committed the laws to parchment - the Twelve Tables of the law.