Two ways it's better: it sets the expectation with the sender that you won't get an immediate response, and it provides a better option for those without the discipline/self-control
From what I know, you cannot fine-tune your DND. For instance, I want certain notifications to show up versus not.
I prefer "Cellular Data Off," because there is greater need distance that way.
Android has a few filters: alarms only, priority notifications, calls/messages from specific contacts, etc.
I agree that greater flexibility in creating custom rules would be welcome though.