Sometimes you need to comment out a line. Perhaps a line is meant to be commented out most of the time, and put back occasionally for debugging purposes. Or maybe a program is misbehaving, and removing a line might prove enlightening.
In any case, the need sometimes arises.
At such moments, it is important to remember what language you're using.
// doesn't work for Ruby nor SQL.
# doesn't work for C, PHP, nor SQL.
-- doesn't work for C, PHP, nor Ruby.
And, if it's in some, e.g., PHP script that gets invoked by Someone Else... it may take a while for the problem to show up in the server log.
Maybe I should ditch all these different languages, and write everything in LISP. Or APL. Or FORTRAN. Verilog, perchance? Is there a mod_cobol for Apache?