What is the function(s) of the hidden Debug menu in Disk Utility? What is the function(s) of the Debug menu and why is it hidden by default?
The Debug menu mostly gives access to functions in relation to hidden partitions (e.g. the system-restore-partition in Lion).  see Debug Menu 

Most likely the Debug menu is mainly for Apple internal use (their techs dealing with stuff probably found these functions useful) so it's hidden by default.