I think the reason you see it a lot here, is that it is very easy to show what is going on when file processing questions are asked, ....

Yes.   Oftentimes we are trying to help posters who are having quite enough difficulty grasping basic Perl syntax, without asking them to understand why setting up a data structure and manipulating in a particular way is 'like' reading a file.   Using __DATA__ is the _gentlest_ way of helping them to understand the point of the blessed reply, without looking like we are cursing at them in perLatin.