Why crowbar is needed?
SCR Crowbars are usually used in systems which should interrupt permanently the normal operation in case of a fault (e.g. Over voltage). In you circuit, the normal operation of the circuit should be permanent interrupted if large abnormal over-voltage transients arise in the load side. In this case, the crowbar makes sure that the power supply's output is short circuited, and its input fuse is blown, thus interrupting the flow of the current.