Showing up the notification icons on android status bar is very helpful for you to manage the pending items.  e.g. unread messages etc.

So you don’t have to swipe down everytime to see whether there are any new messages, emails etc.

To set (this settings is based on Android version 6 marshmallow)

1. Go to Settings
2. Choose “Notifications & status bar”
3. Enable “Show notification icons”