Many software teams create services exposed as APIs, designed to be
consumed by other software and thus without any user-interface. Such
services are hard to demonstrate, as they effectively just dump pages of
JSON. A demo front-end is a simple user-interface just used to
manipulate such an API. Matteo Vaccari describes how and
why to build one – showing its usefulness both in explaining the
API’s capabilities to stakeholders and to help client developers explore
how to interact with the API.
Source: martinfowler