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To test your own API while developing and before making that API live on the server of your organisation.
To get feedbacks feedback and bugs quicker.
For checking the dependencies in your API before it is available to everyone.
For QA engineers to use this for testing/isolating external dependencies and integration testing.
By front end developers to use it before actual endpoints are available. This is done while designing the UI so that you don’t need to wait for the time till actual endpoints are developed. It saves a lot of time.
For engineers to develop prototype of their idea to present it to the investors for funding.
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