Basically, dispatcher=true means "consider this method as the beginning of a request that I want to control separately from other types of requests."
From the documentation for monitoring Java background processes using a new relic :
The dispatcher=true parameter causes the annotated method to be processed as a web transaction - the agent will report errors and transaction traces for the task.
Set to false , the traced method will not be considered as the transaction itself and will be displayed only in New Relic if it is called from an already reported transaction.