If you trust all the people who can add content to your site (i.e. if adding new nodes, comments, etc. is restricted to registered users who you know and trust), it may be convenient to set "Full HTML" to be the default text format, particularly if you're using a WYSIWYG editor whose functionality may be partially blocked by the "Filtered HTML" format, if you haven't spent a lot of time adding a lot of HTML tags to the list of acceptable options.