The project's stated purpose was practical. To recognise idolatry, and to detect it surviving under Christian forms, missionaries needed to know what it looked like, and Sahagun argued that a physician must know the disease. The work therefore exists because the church wanted an inventory of what it was suppressing, and its completeness is a function of that aim.
The plan was not welcomed indefinitely. Authorities became uneasy about a detailed account of a suppressed religion circulating at all, the materials were ordered collected, and the manuscript ended up in Europe rather than in use in Mexico. The result is that the fullest record of Mexica religion survived by being removed from the place it described. Both facts have to be held together, and neither cancels the other. The codex is the single richest source for Mexica deities, festivals, calendar, medicine and formal speech, and it is a colonial instrument whose questions shaped the answers it received, whose organising categories were European, and whose informants were speaking under conquest to men who held power over them. Its twelfth book, an account of the conquest given from the defeated side and revised over the years that followed, is the clearest single place to watch both of those things operating at once.