As an illustration & animation artist myself, I have come to understand (over the course of 25 yrs.) that in most situations where you will be drawing/painting in any number of diverse types of mediums, one must understand and be proficient in the use of color's (in their respective format, be it traditional hands-on or in a digital form), the tools (brush's, ink's, stylus', etc.), anatomy of the human/animal body (in order to properly and convincingly depict believable characters, be it fictional, realistic or a stylized middle ground).
You can't create an alien plant if you don't know which general shapes most organic plants on your own planet come in (helps with the imagination). Photos help train your eyes (long-term memory) with proportions, distance(s), perspective(s), surface types of different objects (glossy, matte, rough, jagged, etc.), Toy's help you visualize how an object might be able to move or articulate to better represent the object it is made to represent and in animation it helps visualize how you want and don't want your character to move or deform. Other things like cloth, water, the flow of air, dust clouds, etc. help you understand how things work in the real world or how they move thanks to gravity and mass, and it allows you to depict them properly in your work.
In short, as an artist you must learn to observe, recall information, and then illustrate it in a way that someone else can look at your work and automatically agree that the object(s) shown represent or are recognizable to them as what you want them to think they are...(Ah, yes, this is an apple sitting on a picnic table with a table cloth draped over it) and the sun is hitting the varied materials with the proper light properties your brain recalls them to be from previous encounters with these objects in the real world.
