In a nutshell, themes are sort of the 'point' of everything you do. For example, when you pick a career, there's probably an underlying reason that goes much deeper than a paycheque and what areas you feel there are jobs. As a writer, I like making people think about things that perhaps wouldn't normally cross their mind. There are tons of other reasons but that would be the 'theme' of why I choose to write.
In my books, themes are essential because they are the lifeblood of the story. I think the best way to demonstrate this is to give some examples.
In my first two books, Fire and A Spark before the Fire, my theme was how our culture often doesn't see entertainers as real people but rather machines who's every move is meant to entertain us, their lives on constant display, kind of like an animal in the zoo. This was brought on when I watched as a celebrity had a very public meltdown and people were essentially laughing at her, rather than having any concern for her mental state. It frustrates me that we have tabloids and silly shows that focus on everything the stars are doing and actually, this has only grown worse since writing these two books. Of course, this wouldn't exist if there wasn't a demand.
My final books turned into an extended series that easing the reader into the whole world of organized crime, criminals and an underground world that most perhaps assume is a rarity, if they even believe it exists at all. The central theme is corruption, crime, collusion and the belief that sometimes it's hard to be certain who the 'good' and 'bad' guy really is; although my books follow a group of criminals, there is a point where readers see that the lines are often blurred. My goal was to make readers think and perhaps ask some questions, if only to themselves. Who really runs the world? Politicians, corporations, religious leaders....or is it you? Who controls what we see and what we know? Is it manipulated? These are the questions that I've explored as the series unfolded a couple of years ago and I continue to ask myself.
I think themes are as important for the writer as they are the reader. It gives us a focus that is bigger than simply telling an entertaining story but something that might have a bigger message.