I am a doctor. Every time I see a patient who also sees other doctors, each of them has to get my note ... and visa versa. There may be many businesses where faxing is becoming obsolete ... but medical offices are not one of them. We get 50-70 non-garbage faxes a day.
Image conversion is a critical step before faxing. Since printpreview creates an image of the document, it is the logical place to have printpreview specific manipulation of the document before submitting it to winfax.dll.