Usefulness of research paper(s) of another discipline for PhD Consider the scenario in which a PhD student of the subject X published good research papers on subject Y. X and Y are not related. Will those papers any way useful for getting PhD or any benefits related to PhD?
It is unlikely that any papers in a different field will be used in assessing a PhD. However, as long as the papers are not too time consuming to generate, then the process (doing research, writing it up for publication, responding to reviews) will be useful for doing the work necessary to get your PhD. And more papers will always help in applying for postdocs.

Depending on the subject, a PhD can be more about learning to do research than obtaining subject matter knowledge. If you are in such a discipline, then research practice is useful. But it is counterproductive if doing the extra papers reduces your capacity to do research in your own field (particularly time).