Daydreaming is extremely important to mental processing. It's not just spare cycles firing off randomly, your brain is designed to spend lots of time basically operating your body on autopilot and using the spare resources to collate, analyze and integrate information that has come in since the last cycle. Making sure you get enough DMN/daydreaming time is important, as is recognizing when the DMN is spending your spare cycles on things that aren't useful to you. For example, trauma survivors often manifest symptoms that are linked to a hyperactive DMN. The thinking is that in a threatening environment there's a pretty direct payoff to being hypervigilant and focusing on finding threat patterns and that maladaptive behaviors start out as adaptive behaviors in a different environment.
tldr - Doug Funnie was actually the model of a healthy boy with a rich inner life