The argument is that comments are often used as an escape hatch from specified formats to carry further instructions. So you got a properly specified format and

The argument is that comments are often used as an escape hatch from specified formats to carry further instructions. So you got a properly specified format and then want to do vendor&extensions but not break other implementations ... just make your extensions a comment. Then other parsers ignore it and you can do your thing. The idea is that this forces better formats. How well this works? Well, then I got an "x-comment" property or non-standard comments. Nonetheless. If people see the need to hack some extension in, they'll find a way.