They were begun (if I recall correctly) by one of the Roosevelt administrations.Before trump, how frequent were press conferences?
Rose Garden - White House Museum
They started holding press conferences in order to stay on better terms with the press and to have better press. They could invite the press members they wanted, etc. I don't remember all the details, but that was when the tradition began. Its not required any more than the pardoning of the turkey.
I think the idea is that if the press is denied any information they start spreading rumors, so the Rose Garden is intended as a kind of control. I'm not sure how good of a control it is, but it does help presidents please the press and make friends in the press.