There’s one reason I track the source of where I obtained a genealogical record: myself. Today I was looking at a digital image I had of a record. Because I sourced it, I knew that I had made the digital image from a copy of the microfilm at the Family History Library. That’s different from a scan I might make of a photocopy I made at the courthouse years ago, different from a scan I made of a paper copy I made from a microfilm printout, and different also from a picture I may take of the actual record.
All of these have differing levels of detail from the document. If I don’t know where I got it, it makes it difficult for me to assess years later whether it might be worth getting a copy of the original.