Hart (1998) explains the usefulness of citation mapping and analysis in order “to map out the development of an idea, technique or theory“, which is an important aspect of a literature review. See also what Wikipedia has to say about citation index.
Hart C. 1998. Doing a literature review : releasing the social science research imagination. London: SAGE.
Noruzi A. 2005. Google Scholar: The New Generation of Citation Indexes. Libri, International Journal of Libraries and Information Services 55(4):169-253.