comment-dwim on blank lines

[Glenn Morris]

If comment-dwim is called on a non-blank line, it calls comment-indent
to do the work of either inserting or indenting a comment. On a blank
line, it does the work of inserting a new comment itself. Is there a
reason for this difference in behaviour?

It means that there isn't really any way for modes to customize what
comment-dwim does on blank lines, other than by abusing
indent-line-function. In particular, comment-insert-comment-function
does not get called, which seems to contradict its doc-string.