Generally, the backend is developed first before the frontend after establishing the API contracts. However, in some cases, they may be worked on together (in parallel). This is especially true in case of tight deadlines (like a hackathon).

Or consider a case where you're working on both frontend and backend yourself. And you want to get the frontend up first so that you can float it around to get feedback.

In these scenarios, having the ability to quickly set up a dummy backend helps. You could potentially define the response type (JSON, XML, HTML, CSS, etc.), headers, and the content (body) to be returned when you hit a particular URL.

⬇️There are a bunch of them - my favorite is Mocky.