Journals are used as a proxy for quality. A publication in "Science" or "Nature" is generally regarded as being better than a publication in, say, the "Proceedings of the National Academy of Science".
Bibliographic metrics, like the number of publications and where they are published, are use as a proxy for the quality of someone's research. It can affect grant funding and career advancement.
Researchers can publish their own papers. But researchers also tend to want that work to be disseminated, both to spread knowledge and get recognition for the work. Journals help simplify that process. A field typically has one or two main journals, which most people in the field track. It's more likely that a publication will be noticed if it's published in one of those papers.
Bibliographic metrics, like the number of publications and where they are published, are use as a proxy for the quality of someone's research. It can affect grant funding and career advancement.
Researchers can publish their own papers. But researchers also tend to want that work to be disseminated, both to spread knowledge and get recognition for the work. Journals help simplify that process. A field typically has one or two main journals, which most people in the field track. It's more likely that a publication will be noticed if it's published in one of those papers.