I'm actually really curious, because reading the pdf linked elsewhere in the thread (on acceptable variants of the (c symbol) specified that stating copyright was optional for literary works, which implies it is not optional for eg code.
> “Literary works” are works, other than audiovisual works, expressed in words, numbers, or other verbal or numerical symbols or indicia, regardless of the nature of the material objects, such as books, periodicals, manuscripts, phonorecords, film, tapes, disks, or cards, in which they are embodied.
The reason to have this category is mostly to distinguish it from audiovisual works where there's a developed system of compulsory licensing around stuff like paying royalties for playing music on the radio or in public venues.
This point has been litigated too, e.g. in Apple v. Franklin. https://en.wikipedia.org/wiki/Apple_Computer,_Inc._v._Frankl....