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seo_experts - 10:43 am on Aug 19, 2006 (gmt 0)
The reson was to let the visitor know that he/she has already visted a link and not visit it again. Sometimes you are too lost in reading or seeing that you loose sight of what you have already visited and what is left to be visited.
It has been a general practice since time immeorial to make visited link different then the rest.