Citation analysis can be used to evaluate the impact of a scholar's work as well as the importance of a journal in a field. While judging the quality of an author's body of work, an article or a journal can be subjective, impact factors can still help a scholar judge where it might be best to submit their own research.
When you are developing your tenure portfolio it is a good idea to be able to justify your scholarly impact by presenting the number of times each of your publications have been cited by others (and in which journals). Journal and article level metrics can show the importance of the journal where your articles have been published. After tenure, it is a good way to continue to track the impact of your ongoing scholarly career.
No one source will have all the information about who has cited your work. For each of your publications you will need to search the main citation tracking resources to ensure complete coverage.