What is the purpose of the defence of qualified privilege?

The purpose of the immunity provided by the defence of qualified privilege is described in The Law of Defamation in Canada, at pp. 13-15 to 13-19 (footnotes omitted): The protection ... is justified on the basis of public policy and utility, and in furtherance of the “common convenience and welfare” or “general interest” and “advantage” of society. ... The purpose of the immunity is not so much to protect the parties involved as it is to promote the public welfare. ... As Bankes J. in Gerhold v. Baker [[1918] W.N. 368 at 368-369 (C.A.)] said: “It was in the public interest that the rules of our law relating to privileged occasions and privileged communications were introduced, because it is in the public interest that persons should be allowed to speak freely on occasions where it is their duty to speak, and to tell all they know or believe, or on occasions when it is necessary to speak in protection of some common interest.”