Meh, would be my typical response. Besides a beautiful breath taking cover I want no other suggestions of what my characters are going to look like. However, if a graph will help demonstrate something that can not properly be communicated in words then I think it is essential to be included.
Typically it depends on what type of book I am reading. If it is fiction , the fewer distractions the better. Unless of course it is Brian Sanderson and his complicated magic system. Then I might need some visual aids.
A children’s picture book requires pictures. If I was picking up a traveling guide, I would expect pictures and maps. A graphic novel the same thing. A book explaining architecture, I would love indepth photography for it. If it is appropriate for the medium of information that is being presented than by all means included. If is superfluous then don’t.
However, as for my fiction, I prefer absolutely not distractions besides the words on the page. I am there to create my own concept of the world, not someone else’s visions. And by someone else, I mean someone who is not the author. Author’s jobs are to paint with words. If they can not do that, then they are not doing their jobs properly. However, if you were to say give me a link to a website to explore at my own leisure, I might be inclined to do so. Just not in the book. Keep that material separate.