Journal Rankings are used as a way to see what titles are the most used within a field or discipline. This ranking is based on many factors such as the number of times that a journal has been cited, the quality of the journal that cited the title and the journals visibility within a discipline. Knowing this helps us understand the scholar's impact in their chosen field.
Journal rankings also help an author decide if a journal is a good fit for their article. They can see if the review process for the journal is done by an editor or if the publication is peer reviewed. Other characteristics of the review process would include how many reviewers and whether the reviews are "blind". Finally, what percentage of submitted manuscripts are accepted by this journal? A high percentage might indicate a title that is easier to publish within and a low percentage might mean the journal is very selective.
Journal rankings are important and in some schools are used in tenure decisions.
From the SciMago webpage: "The SJR (Scientific Journal Ranking) is a size-independent prestige indicator that ranks journals by their 'average prestige per article'. It is based on the idea that 'all citations are not created equal'. SJR is a measure of scientific influence of journals that accounts for both the number of citations received by a journal and the importance or prestige of the journals where such citations come from It measures the scientific influence of the average article in a journal, it expresses how central to the global scientific discussion an average article of the journal is."