I agree with what you have said, I would like to add:
The use of photographic images can be of great help to an art student or an established artist, but, it is most important that a deep knowledge of photography is known before you start taking photos and drawing pictures (or painting).
It is most important that when you take a picture you do not distort the image by your photograph; the extreme distortion is with the use of wide angle lenses, in general you should not use anything under a 110 mm lens if you want to keep things in proportion.
Taking several pictures of a portrait subject gives the artist time to study the best angles (and keep a record) of the person to be painted. It also gives extra time to the artist to do the painting and relieves the subject from posing for hours while the portrait is being made. A one "last sitting" is always wise and a good practice.
For a landscape, it gives you the chance to add or delete and make your own compositio; apart [from] that the artist is relieved of wind, flies, mosquitoes, etc.
A traditional oil painting is not an oil photograph. It is the artist's interpretation of the subject and it may be different from the original view.
Albert