This could as be a hypothetical use case, however, I found myself needing this:
Take a pdf file, PDF1, originally created from say an MS Word document WD1, and then create a tex source file, TS1, that can also produce the exact contents of PDF1.
This is somewhat hackish, but if tex can render images linked to in the tex source, then perhaps it can also render linked pdfs. Thus, where a pdf already exists, the tex for producing the pdf can be authored thus. However, if there is a more direct method of creating a TS1 from WD1 without the intermediate PDF1, that should be a better method.