I was asked to fill in for another illustrator who had trouble with a book cover. So, I redid the job for free. A few days later they offered me my first manuscript. It's about a little girl who helps a neighbor in his wood shop making whirligigs and bird feeders. No, most authors have particular visual ideas, but it s often best to allow the illustrator free rein to think of their own visuals.
Illustrators are, ideally, better trained to come up with more unique images.
That would be the typical case, although once in a while an author knows best. I probably have had authors secretly wish I had done something different, but outwardly they have all been positive. I created charts and maps. One of the most hilarious diagrams I had to do was of Ronald Reagan's prostate. I think the AP ended up being too embarrassed to run it.