When Apple first introduced, it was welcomed as a solution for not interrupting meetings or waking you up all night from alerts.
While DND succeeds at those two claims, immediately after the release of iOS 6 users realized it needed more. You see, on iOS 6 users who had DND turned on but were still using their device would still see and hear alerts while the screen was on. The only time DND was truly silent is when the phone was locked and the screen was off.
For users who want to truly silence their iOS device, even when they're using it, Do Not Disturb in iOS 7 will make you happy.
With the addition of one simple option in Do Not Disturb's settings users can elect to have the device still alert them when the screen is on, or remain completely silent. To get there, go into the iOS Settings, then select Do Not Disturb from the list of options.
Scroll to the bottom of the page where you'll find the Silence section. Here you can opt to have Do Not Disturb silence your device when it's both locked and unlocked by tapping on the Always option.
Again, it's a simple yet effective way of completely silencing a device when DND is enabled, instead of selectively silencing alerts based on whether or not the device is locked.