There are very nearly as many (if not more) rationales for freedom of speech as there are books and articles on the subject. Without attempting to canvass them all, I think that they can be divided into two generic theories. One theory suggests that freedom of speech is essentially teleological or consequentialist, i.e. it exists to serve some other goal, usually effective participation in the democratic process. The other theory, which is deontological or normative, suggests that freedom of speech exists as an end in itself rather than as a means towards accomplishing something else. Of course, these theories are not necessarily mutually exclusive.
Arnold H. Loewy,
Freedom of Speech as a Product of Democracy,
U. Rich. L. Rev.
Available at: https://scholarship.richmond.edu/lawreview/vol27/iss3/4