This kind of turns the API into a client application, that is, you do not necessarily need to build a user interface on top of the API, it is actually inbuilt in the API.
The main customer support application in a IoT company that I run is just such an API. The customers themselves of course are offered a real app on top of this API, but not having to maintain a dedicated application for the support department is a huge saving.
An example with Microsoft's Northwind dataset:
http://arnorthwind.agorapide.com/RQ/dt/Employee/1
gives HTML output with helpful links, in this case related EmployeeTerritories and related Orders.
The same query in JSON
http://arnorthwind.agorapide.com/RQ/dt/Employee/1.json
will show data for the Employee only.
This kind of turns the API into a client application, that is, you do not necessarily need to build a user interface on top of the API, it is actually inbuilt in the API.
The main customer support application in a IoT company that I run is just such an API. The customers themselves of course are offered a real app on top of this API, but not having to maintain a dedicated application for the support department is a huge saving.