"Use of the document structuring conventions... allows PostScript language programs to communicate their document structure and printing requirements to document managers in a way that does not affect the PostScript language page description"

The idea being that those document managers did not themselves have to be PostScript interpreters in order to do useful things with PostScript documents given to them. Much simpler.

For example, a page imposition program, which extracts pages from a document and places them effectively on a much larger sheet, arranged in the way they need to be for printing 8- or 16- or 32-up on a commercial printing press, can operate strictly on the basis of the DSC comments.

To it, each page of PostScript is essentially an opaque blob that it does not need to interpret or understand in the least. It is just a chunk of text between %%BeginPage and %%EndPage comments.

This is tremendously useful. A smaller scale of two-up printing is explicitly mentioned as an example on p. 9 of the spec.