Why do we do visarjan on the last day of ganpati festivals?

Answer 1

I'm not sure but maybe there's something in one of these links:

http://www.sscnet.ucla.edu/southasia/Culture/Festivals/Ganpati.html

http://www.geocities.com/qqplus/ganpati_festival_in_india.htm

http://en.wikipedia.org/wiki/Ganesh_festival