Ad-hoc start event is a very handy in IBM BPM . The obvious use of this event on BPD is that to launch a human service or system service, which is not connected to actual workflow, on demand such as to terminate the instance. For advanced users it is a way with which ad-hoc tokens can created in a given process instance for the purpose of token injection. Sometimes in a complex workflow process user might want to resuscitate a closed task/activity and recreating the instance is not an option. If we know the target activity Id and token number that created from ad-hoc event, using bpmrest-ui tool we can easily inject a token to bring up the desired task again.
This is from IBM knowledge center:
"Important: Ad hoc actions are deprecated in version 8.5.5.0, instead use Creating an unstructured (ad hoc) activity."