What is the rationale for the attorney-client privilege and the husband-wife privilege?

A counsel is not allowed to divulge any of his conversation with his client, unless it concerns a criminal act which is still to be committed, or if his (counsel's) testimony is of such essence that it is necessary to the determination of the case and the proper administration of justice. A lawyer owes it to the courts, to his clients, to the legal profession, and to his brothers in the legal profession to keep his communication with his client confidential, except in the cases aforementioned. On the other hand, the husband-wife privileged communication arises from the fact that the spouses are considered as one, being married to each other. It may not be invoked when one spouse files a complaint against the other, and they are the sole parties to the case (like in annulment or divorce proceedings). The privileged communication only applies to instances wherein one spouse is called to testify against the other (like in criminal cases where they are not the sole parties to the case).