Let's start with an exciting use case. You have implemented your big idea and created a fascinating website or mobile app, and you are ready to tell the world about how cool your app is. Everything went well, everyone wants to use your app. Then suddenly your backend starts to reject new registers because it receives more requests than it can handle. First of all, good for you, it looks like you built a product that people want to use. Secondly, shame on you, because you didn't test the maximum concurrent requests capacity of the backend.