At present an application for an ARC postdoctoral fellowship will not be looked at if an applicant doesn't have at least five referred publications. Twenty years ago successful academics hired in the School of Geosciences had a minimum of eight refereed publications. While a student owns their research, the supervisor often has supported that research through grants. The supervisor is trying to establish a research profile that allows them to obtain future nationally competitive research grants. To this end the supervisor may ask for joint publications with a research student.
Joint authorship is a type of apprenticeship.
The senior author on a paper often has publishing experience, knows what will sell and what will not, and knows how to structure the paper, how many figures should go in and how the material should be referenced. The senior author also may have a presence in the field that can expedite a paper into publication easier than if it was submitted by an unknown person. A fair system of sharing authorship is for the supervisor and the student to rotate the position of lead author on papers. Some supervisors may be more magnanimous and give the student lead authorship on all papers. Rarely will the supervisor insist on lead authorship on all publications.