I used to never think about what my books' themes were, but a year or so ago I realized how important it is, and ever since then, part of my book prep fun has been figuring out what the main theme of that particular book was going to be.
Now for some clarification. A book's theme = the main message throughout the book. No matter what genre your book fits in, it's going to have some sort of theme. Without a theme, your story will just be a bunch of things that happen without any compelling connection to the main character. The theme is the heartbeat of your story--you may not always hear it, but it's what makes everything go smoothly. ;)
Some examples might be: Pilgrim's Progress--A journey (spiritually, inside of Christian, as well as a portrayal of the journey of life), Little Women--Family (the importance of family, family relationships growing stronger etc), The Bronze Bow--Forgiveness and Love (Love for Jesus, love for his people, and love and forgiveness for his enemies).
So those are a few examples, and I'll just say here that people will probably interpret different themes from the same book sometimes too. :D But still, having a plan of what message you want to run subtly--or obviously--throughout the book is a good idea so that you do have a specific thought in mind as you write.
What are the theme(s) of your book, or from your favorite book to read? This NaNoWriMo, my book carries the main theme of accepting God's will for your life.
You all have a lovely day!