In a democratic society, the rights of one person will inevitably conflict with those of another. And the potential for conflict between photographers and their subjects looms large.
Photographers have rights to free expression, but individuals have rights to privacy.
Whose rights prevail under the law depends upon how a photograph is to be used. As a rule of thumb, the photographer's right of free expression usually prevails when a photograph is used in an editorial context. Such uses don't require the subject's permission. But rights of privacy prevail when someone's image is to be used in trade or advertising, so the photographer has to get permission. These releases oil the wheels of business by reducing the potential for conflicts that end up in court.