When it comes to getting various notifications while your device's display is off, a built-in LED is useful. However, it's far from perfect. Sure, you can customize each notification with something like Light Flow, but that doesn't let you know the message contents at a glance. And, of course, there are also devices that don't have an LED at all, like the Nexus 7 for example. If you're looking for more out of your notification experience, Knock²+ may be the answer.
Update: Apparently the app isn't compatible with the Nexus 7 right now, but the dev is working out the kinks and should update the app "soon."
Like others before it, Knock²+ essentially provides a lockscreen popup when a notification is received, so you need only glance at your device to see what the ping was about.