JSON Helper has no interface, and runs in the background waiting for AppleScripts to ask it to do something, just like Apple's own System Events does.
JSON Helper contains a number of commands that help you parse JSON into regular AppleScript lists and records, and convert AppleScript list and records back into valid JSON. In addition JSON Helper contains some convenience commands to allow you to communicate easily with web based APIs.
To see how to use JSON Helper, check out these examples.
Why is JSON Helper not a Scripting Addition?
A popular way to extend the functionality available to AppleScript is to make that functionality available in a Scripting Addition (or OSAX). JSON Helper is not implemented as a Scripting Addition for a couple of reasons. The first is that Apple appears to be actively discouraging the creation of new Scripting Additions. The second is that it's currently not possible to distribute Scripting Additions via the App Store.
Agents do have one advantage over scripting additions, which is the commands they provide do not pollute the global namespace of AppleScript, instead they are available only within a script's tell statement that targets the agent.
Will it ever be a Scripting Addition?
Maybe! If there is enough demand for one, then I'll try and provide it as a Scripting Addition as well. Donations via PayPal would be a good way to encourage me!