This is the functionality which when enabled from Admin panel allows the visitor to view old messages in chat interface.
The functionality gets auto disabled when a visitor clears his/her cookies or cache. In such a scenario, visitor will not be able to view the old messages.
In order to enable asynchronous chat functionality go to Settings>Common>Campaign and the following screen will be redirected by the system as shown below-
Click on the campaign to view the skill grid like the one as shown below-
Click the button to enable/disable asynchronous chat functionality as shown highlighted in the screen below-
A visitor connected to the asynchronous chat enabled skill will be able to view old messages in chat interface unless cookies or cache of the browser have not been cleared.