This is from Aichele’s Sign, Text, Scripture; it is obvious, but still well put:
What makes the Christian Bible a canon is the decision of a Christian group that a specific list of books, and only the books on that list, count as the ‘Word of God.’ These books ‘speak’ with the authority of God for those who belong to that group. Furthermore, the single physical volume—the biblical codex—connotes to many people that the Bible is a single book and that it consequently bears a single consistent message. The canonical codex signifies more than just the authority and sufficiency of the message attached to the designated text; it also signifies a fundamental unity in the message’s content. At the very least, the canonical messages cannot contradict each other. Canon signifies that all of the canonized texts themselves signify essentially the same message. This encourages intertextual play between the canonical texts on the part of their readers. Thus canon both limits and stimulates creativity in the interpretation of the texts. (132)
I like the last line best: on the one hand canonization limits how you can interpret the texts—everything you say or read out of the text should in principle fit with the rest of the stuff in the canon—but on the other hand this is exactly what inspires creativity! As changing social circumstances require new readings of the text, the more creative social groups become with making the canon do the work they want it to do without appearing as if they are doing so.
Having a canon is like having someone lock you up in handcuffs, take away the key, and then say: “Now how are you going to get out?” You’re limited but you are simultaneously forced to be creative.