Example:
1 Database for multiple programs:
Vbulletin & Wordpress
Both have a table called settings, without a prefix, the 2nd program isnt going to complete its install as MySQL will tell you that the settings table already exists & it wont create it.
Adding a prefix to either programs db tables will create instead two tables:
Vbulletin = settings
WP = wp_settings
Both programs will now be able to share the same DB without MySQL complaining that the settings table already exists. So WP would be set up with a prefix of wp_
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