Use it when you are trying to see just what kind of gunk^Wdirectory paths %INC is filled up with, when you know you have installed a module but perl is failing to find one.
Actually, in such a case
@INC is far more useful than
%INC.
%INC just gives you information where Perl actually has
found a module it loaded - it doesn't give you much useful information if it couldn't find a module.
Other similar use is when trying to sort out the module paths, say to clean up the duplicates.
Again, in that case,
@INC is far more useful than
%INC.