In general, with any flash that has Auto mode as well as a TTL flash mode, the Auto mode is useful if mounting the flash on a lightstand and putting the unit remotely and camera-controlled flash exposure is not possible. In the days before wireless TTL flash control was possible, this was frequently true.
In the specific case of Canon-brand flash, unfortunately the engineers at Canon designed External mode metering to severely UNDERexpose shots...by as much as -2EV! This is well documented on POTN. This was true in the 580EX and unfortunately has proven to continue to be true with the newer 600EX as well! So it is necessary to LIE to the flash about ISO or f/stop used (or use FEC), in order to get it to deliver a sufficient amount of light onto the scene.
In a number of specific camera+flash combinations, Canon camera-controlled ETTL would underexpose and need FEC, and/or ETTL might unpredictably mis-expose a shot...in which case some of us would resort to known-consistent and accurate Auto flash like Metz, even though they were ETTL compatible units. For example, ETTL seems prone to issues of full power flash output due to somewhat questionable connectivity between flash and camera, and Auto flash is not subject to such unexpected flash output errors!