If you’re designing a product or application where people need to notice an event that rarely occurs, use a strong signal to get their attention when it does.

For example:

When you're on your phone you may not be paying much attention to the battery indicator all day, but it Is going down. Eventually your phone will remind you when you're at 20% and if you're anything like me, you'll ignore that and move on with life but then it reminds you again at 10%.

Your phone sends you a "strong signal" to get your attention when it is going to die soon.