Suppose you are a top class rapper and you haven’t released an album for a while and fans are asking for it day and night. So what you do is that you “promise” them that whenever it will be out, all of them would be notified. To get this done you give your fans a list. They can fill in their email addresses, so that when the album becomes available, all the subscribers instantly receive it. And even if something goes wrong, say a pandemic, so that you can’t release the album, they will still be notified.
Now everyone is happy: You, because the people don’t crowd you anymore, and fans, because they won’t miss any news on the album.
This is a real-life analogy for things we often have in programming: