The purpose was never neutral and Sahagún never pretended otherwise. In the prologue to the first book he compares the friar to a physician who cannot prescribe without first knowing the disease, and the disease is the religion he is describing. The Franciscans had found that converts baptised in the 1520s were still keeping the old calendar, still making offerings, and still using a vocabulary whose meaning the clergy could not hear. A full description of Nahua ritual was a working tool for detecting it.
The Spanish Crown understood that too and did not like it. In 1577 Philip II ordered every copy and draft gathered up and sent to Spain, on the reasoning that setting down the old religion in the old language might keep it alive; the manuscript that reached Florence, and gave the Codex its name, survived by accident of that dispersal.
None of this makes the work untrue, but it shapes what is in it. The questions asked were the ones a friar wanted answered, the elders answering knew what the answers were for, and subjects a Christian reader would find scandalous are handled at a careful distance. Mesoamerican studies rest more heavily on this book than on any other single source, which is why the conditions of its making are now taught alongside its contents.