Multiple photos are extremely useful for showing different angles of an animal, such as belly shots for het markers. They're also great when posting multiple animals for sale in a group or individually.
In addition, sellers can now add a caption for each photo. This is already being used to specify date or weights for each photo (as seen above) to showcase animal growth. Another use of the caption is to explain additional pictures which are of the Dame and Sire of the animal.
Buyers typically ask for additional photos anyway, so why not post them on your ad where they'll help to generate more inquiries and close the sale faster.