Transformation are useful to manipulate the preview of the stored documents in a persistent manner, for example some scanning equipment only produce landscape PDFs, in this case a default transformation for that document source would be “rotation: 270 degress”, this way whenever a document is uploaded from that scanner it appears in portrait orientation. The transformation remains attached to the document, this way the file is preserved in it’s original state (a requirement in legal environments) but only the representation is transformed to make it look right to the user.