Let's make one thing absolutely clear. I am trying not to list obvious rewards. Let's assume that moral and intellectual uplift, unexpected insights, rollocking entertainment, novelty, brilliant authorial style, acceptable grammar and punctuation are all givens. Let's look for things that haven't been said before in waffling abstractions,
A book can confirm your prejudices. And the darker and more contemptible such prejudices are, the more thrilling it is to find agreement. One assumes Mein Kampf raced a lot of motors - not mine or yours, perhaps - and a number of readers enjoyed reaching page 720. You may be waiting for a novel which turns dark chocolate into a religious experience; I frankly am not.
Books - often the more rubbishy the better - may separate you from the rest of the human race. Especially in dentists’ waiting rooms. It is not for me to say whether this is a worthy aim. All I can suggest is that recognised masterpieces are less likely to do the job. George Eliot's Romola (definitely not a masterpieces and not fun either) would, for instance, offer little help.
Appearing to read difficult books confers status. Many people holding soft-science degrees, never having got past Northanger Abbey, managed to advance their standing by buying A Brief History of Time. But not for long. A very brief history.
Not all so-called difficult books turn out to be difficult. Proust's opus is comedic. Having cracked this nut you can dine out on the achievement. I know. I have.
Re-reading children's books and talking about the plots in a remote academic way is easy to do and unsettling for those forced to listen. Graham Greene spoke about Beatrix Potter's genius though it's rumoured he did it for the money.