Maintain relationships, represent positions
There exist numerous theories on the development of public relations, their definition and meaning from the past and present points of view. For me personally, communications need to follow an objective.
If Public Relations are primarily aimed at public stakeholders and want to reach political, social or industrial sub-publics, maintaining relationships is of crucial importance in area.
It is well known that people listen to those who have something to say. Hence, the content to be conveyed remains central and can be sourced from raising voice as an expert or thought leader, by outlining visionary approaches, providing opinions, and taking positions that a company or individual stands for, highlights, and substantiates.
For a more theoretical explanation, I recommend the outlines in the chapter "Organization and Communication: Integrative approach to a theory on public relations and public relations management" by Prof. Peter Szyszka in the book "Theories of Public Relations – Foundations and Perspectives of PR Research", published by Springer Verlag (provided in German). Or, of course, the outlines formulated by the PR Suisse Association.