Why? Because the wrong words can paint the wrong image. The image itself, however, shows what ‘is’. If I want to describe a dragon in a story, I need to choose the words that will lead the reader to see in their mind the same (or at least very similar) image that I see in mine.
If I could draw, I could sketch out what I see in my head, but for me that isn’t an option. That is one of the benefits of the internet to writers today. If Tolkien wanted a visual reference of a dragon, he had to go to a library and dig through endless books searching for whatever had been drawn and published. Writers now can simply do a search for images and be handed a tremendous variety to choose from. It can be far easier to describe a dragon in great detail if you aren’t simply trying mentally imagine those characteristics, but can actually see what you want.
That is not to say that you should take someone else’s picture and simply use the dragon you see in it. Your dragon can be a composite of many dragon pictures. Some dragons are given wings, others are not. Some have a smooth snake-like skin while others have scales. A picture can give you an idea of how light might strike a dragon’s scales and be reflected (or absorbed). A picture can help you decide if you want small, vestigial wings that are essentially useless, or wings that spread so wide to each side that a half dozen houses would be covered by them.
Just as writers are encouraged to look around themselves for inspiration in people, situations, locations and so forth, they should also make use of the vast library that is the internet, brought to your doorstep to peruse at your leisure. Maybe you have a vague idea of your dragon being green in color, but then after seeing numerous pictures, you are inspired to change that to a reddish color and do so for a specific reason. Maybe you thought to make its skin smooth, but after seeing many dragons with scales you might decide you like that better. Dragons can lose/shed scales. Those scales might have properties that are beneficial or harmful in some way. Suddenly you have a whole new aspect to your dragon that you didn’t have when he vaguely had smooth skin.
This is just one example, of course. There could be many other applications. If you’re feeling daring, you could set up Pinterest pages to use in collecting images for reference. (Beware, though, as Pinterest can be addicting and cause you to lose vast amounts of time ‘playing’.) Or you could download images to your computer, or paste them into a word-processing document. And you may prefer to do a search whenever you want a visual reference, but not save that image after you have finished extracting the needed details from it.
We live in a very visual world – make that work for you in your writing.