Dialogflow actions are primarily used to tell your webhook code that a specific intent has fired. Now, you can accomplish the same thing by just using the name of the intent. (In fact, that’s what most of the code samples provided by the Dialogflow team do).
However, using the action has a specific benefit – you can update the name of an intent (maybe you are correcting a typo) without having to go and modify your webhook code.
The Actions SDK is a Software Development Kit (SDK) which is used to develop apps for the Google Assistant. In particular, it provides an interface whereby you can perform specific actions on your Google Assistant device. When combined with an NLU engine (bot framework) such as Dialogflow, it will be possible to build powerful voice and natural language based apps.
Click here to read about the difference between the Actions SDK and Dialogflow.
Client Access Token
The client access token provided a way to integrate with the first version of the Dialogflow API (v1).
Since Dialogflow API moved to version v2, the client access token has been deprecated and cannot be used for Dialogflow integrations anymore.