Some writers might argue that theme isn’t an essential attribute of a novel, but I’m not one of them. I’m not suggesting you can’t write a novel without a theme, you can, I just don’t believe it will be as satisfying.
A theme is an underlying idea that takes otherwise disparate events and binds them together. It adds dimension. The theme doesn’t have to be profound, but it does have to ring true to the writer.
Even thrillers or mystery novels are made better by having an underlying theme. Take, for instance, The Bourne Identity. What makes this thriller stand out from others? The plot for one, but I would suggest it is also because of the underlying themes.
One theme of Robert Ludlum’s book might be redemption; here is a highly trained assassin who questions the morality of what he has been trained to do. There is also the theme of the apparent evils of unchecked power. Without a theme as a binder, a novel becomes just a series of events presented in a, hopefully, interesting arrangement. But the events of a novel must add up to something, otherwise, while it might be a satisfying read, it doesn’t stick with you for very long.
Theme is hard for some to grasp. It is an abstract concept, not something you can easily put your finger on. One might not be able to immediately define when it is present but it is definitely something one notices if it isn’t—at least I do. Why should I care about what you are telling me? I ask. If you can’t give me a good enough reason, I might just say, so what?