Journals are regular publications that contain articles on current research. Usually the author of a journal article will have carried out some primary research; perhaps they have carried out a case study where they have interviewed or surveyed participants, or they may have carried out a clinical trial. The purpose of the article is for the authors to outline their research and present their findings and conclusions. This in-depth information is not likely to be found in books and this is why journal articles are important in finding supporting evidence for assignments.
Also, because journals are always published at regular intervals, for example, weekly, monthly or quarterly, the information they contain is very up-to-date. Books on the other hand can sometimes take up to 2 or 3 years to be published.