General considations
Diversifying your research output is an effective strategy to improve your research profile and outreach. Publishing in one journal usually limits the type of research you will produce, as every journal has its own unique aims and scope. Further, journals differ in the type of papers they publish, such as standard research papers, review papers, and technical papers. The advantage of publishing different types of research papers is being able to communicate to different audiences, from theorists to technically-minded to quantitative or qualitative ones. Therefore, publishing in multiple journals will ensure you cover a wider breadth of research topics and research paper formats. This shows you can tackle a wider range of problems, synthesise material into different stories, and convey your ideas to a wider audience. As a result, academics are encouraged by research institutes to publish in multiple, high-ranking journals. Limiting yourself to publishing in fewer journals may also show you are less capable of communicating to a wide range of audiences, which can affect how others see your research potential and collaborative effort.
Practical example for Life Sciences
Within the life sciences, publishing in multiple journals is a common strategy for improving your research profile and communicating your ideas and results to a wider audience. It also serves as good practice for writing different types of articles. For example, the Journal of Experimental Biology mainly focuses on lab- and field-based research papers of comparative physiology, whereas Annual Review of Ecology, Evolution, and Systematics publishes essay review articles. Being able to write both types of articles, for example, is not only a useful skill in itself, but also an effective means of communicating different messages to diverse readerships.
Practical example for Social Sciences
It is generally optimal to publish across a variety of journals in the social sciences to show that you are capable of writing a variety of article types. In particular, it is common within the social sciences for researchers to write a review article summarising the state of the research in their particular niche. This review article can then form the introduction to the Ph.D. thesis, with the added advantage of having been peer reviewed. Subsequent research and technical papers can then form the body of the thesis, and it is advisable that these be published in more than one journal as well.