What was the strategic purpose of bible when it was being written?

To record their version of "truth." There was no distinction between fact and fiction, they were written to establish the "official" history with the political and religious (again, no distinction) agenda they wanted people to follow. The idea that history should involve accurate facts of what actually happened is a relatively new phenomenon in human culture.

Did the people of the time understand that nuance? I honestly don't know. I assume most of the uneducated masses didn't, which is why the elites wrote that way.