The iparams.json configurations can be tested locally with the actual visual testing page.
Previously, the iparams.json configurations cannot be tested with a visual page and the configuration with the testing values for the fields have to be filled in a JSON file and it can only tested as a custom app or in the production.
Now, with this feature, the iparams.json configurations can be testing by going to http://localhost:10001/custom_configs page which is similar to the custom installation page (iparams.html) testing page. The file (iparams_test_data.json) used to hard-code the configuration values is not needed anymore in the app source code.