When I was in elementary school, I had to read The Giver for school. It was a good book, Lois Lowry's style is easy for a child to read, and the plot was interesting, as were the characters. Later in life my mum bought me a copy of Gathering Blue, Lowry's companion book to The Giver, which was also good. This year I found the third book in the companion series, Messenger and wanted to read it, but it had been years since I'd read The Giver, so I re-read it. Looking at this book from where I am in life, years later than when I had first read it, I find it to be completely different. It's incredibly dark, something I only got a glimpse of when I was young. The community Jonas lives in is a perfect communism, absolute mannequinism as well. When we first read this book, we had yet to be taught what a communism was, we would have never made that oh-so-clear connection.
Things change as you age, you gain more experience to draw on, more connections to be made, and even a better understanding of the vocabulary and topics in a book. Maybe you missed something when you first read a book a few years ago, something that you'll understand now. Maybe you'll love a book you didn't much care for before, maybe you'll see a classic in a new light. Rereading isn't repeating, it's a new way of looking at things, and as a reader, and certainly as a writer*, this is important.
*Why is it important for writers? Because we can get a glimpse at what our readers will think when they read our book--the different ways it can be viewed. Some people will get the message, some won't. Some will like it, some won't. It depends on the reader--but who knows, maybe they'll re-read it later in life and finally understand what you want.