What's the point?

Everyone can write whatever they want.
One might write for the pure enjoyment of writing, without any need for others to read it.  A writer may write with the sole intent of earning money by selling their work.  Writing can be therapeutic.  
The books might be designed to do nothing but entertain the reader, with nothing substantial to think about afterwards.  Novels may also have themes- something for the reader to examine during AND after the reading, such as loneliness, self-awareness, desire, redemption, culture, community, wealth, appearance, hypocrisy, forgiveness, or countless others.  These might even be combined.
Often, I believe, that a novelist will write a novel without being conscious of the theme, but the writer runs the risk of being all over the place, with a disjointed story. 
Because of this risk, I think it's best for authors to stake out a theme before they write.  They do not have to be a slave to it, nor do they have to plot out the entire story in advance.  Pantsers and plotters alike can choose a theme and then let it guide them. Even if authors have a story gel in their minds before they select themes, they might try to identify the theme of the story they've captured, and let it guide them as they wrestle the story from brains to pages.  
Authors know they've done this when they are asked, "What is your novel about?" and they are able to answer with a theme instead of characters and plot points.  

Q: What's your book about?
A: Growth, self-awareness, and redemption.

If an author cannot immediately do this, because they hadn't considered it, it might even be fun to spend time thinking about the book and discovering what the theme is.