The problem with painting from photos, or more commonly jpegs on a screen, is that the software has interpreted the information in a way that it thinks will be best for us. This could involve heightening certain colours, increasing contrast or skewing the perspective. In a sense this just replicates some of the selective processes we make when we look at anything we are going to paint in nature. To look at something really is a creative process.
The leap from a subject matter image to a finished painting involves thousands of decisions that affect the process. In watercolour particularly some of the steps seem counterintuitive or just plain wrong, as tonal contrasts are constantly negotiating their place. On another level we have the weight of expectation on our shoulders, like if we were trying to do a portrait of a family member. Its' natural to see a beautiful landscape and try to capture it with your phone camera, I do it all the time! Unfortunately the result is never the same even though it may trigger a fond recollection.
Painters often refer to photos as source material to waymark the route of creativity over reproduction. People often say I choose weird subject matter to paint and to teach painting also. The troglodyte cave dwelling we'll be painting on the 13th was chosen to make it easier to separate out the creative route and the copying route, which I admit to following too often myself. The idea is that if you are not captivated by the wish to reproduce that image then you will be freer to concentrate on things that really make a painting buzz; an interesting composition, well place contrasts, subtle colour shifts, eloquent mark making etc.
That doesn't mean we'll be able to control all these elements like we would wish, but it does mean that we are focused on the mechanisms that are important to get a foothold in developing as painters.