Regarding what it is:
Valerie Lynn Schrader's "Public Memory and the Television Series Outlander" (2021) gives one of the simplest explanations to your question I can find:
Richard J. Cox's article, "The Concept of Public Memory, and its IMpact on Archival Public Programing,":
Cox, Richard J., Concept of Public Memory...
From the book, "Places of Public Memory: Museum and Public Memorials" (2010) it states that public memory is "collective" and not based on any one persons personal "memories," viz. a sort of group identity issue rather than history: a "remembering in common." An official "memory" to which one might subscribe:
This "Public Memory" has its own "history" and so employs "historians" to study it.
From P.J. Brendese's "The Power of Memory in Democratic Politics" (University of Rochester, 2014):
As to why historians employ it, it is evidently the nature of the academy. John Bodnar in his 1992 book on public memory entitled "Remaking America", suggests that studying the history of public memory was a means of making "social history" more attractive to scholars in the 1980s by making it useful to the public sector:
He notes that his own pioneering work on the subject was funded by a number of agencies thanked in his preface, etc.
David W. Blight, advocates for "History and Memory" and the necessity to serve a public interest. He argues for the process:
Historians and Memory...
Here historian Niall Ferguson argues for a restoration of general history, at least in academia.
Niall Ferguson: Decline and fall of History, Youtube