The most common usage scenario for repeaters is as a cable extension. As the length of cables for high-speed data transmission is limited a 1394 connection can only be extended by either using thicker wires or be adding a signal repeater. The latter is most effectively done via a physical layer device as it is the most effective form of signal reconditioning.
The following chart shows a summary of the relation of transmission speed, cable diameter and attainable length between nodes. For more details please refer to the SAE Standards and associated documentation.
Should the requirements exceed the permissable cable length and/or working with multiple repeaters is not feasible then converting the copper based cable extension into optical offers an alternative. Please see DapTechnology's MIL1394 Media Converter product(s).