Because being a child is being an explorer, and grown-ups have the duty to help them on their way.
Also: a book is a small world. If you share it with a child you will learn to know each other as equals, readers both, thrown out of the house and into an adventure together. It might make things much less surprising when they start telling you what they think of your own world.
It's also important for adults to know that children might not agree with your taste in books, and realise that that is a good thing, too. Children need things to disagree with. In fact, your child might have some books to share with you that'll make you think.
Myself, I'm reading a Haruki Murakami book at the moment ("Dance Dance Dance"). I went off his writing for years because the female characters annoyed me, but I'm very much back on track now, thinking about copying out some of my favourite sentences, like one about the protagonist's life being like a squirrel asleep with its head against a nut in winter.
You should be getting MORRIS, THE MANKIEST MONSTER, illustrated by my studio mate Sarah, because it's the most beautifully disgusting picture book ever, and it's just out!