Hi Tom,
"Are the buses all the doing the same job essentially?"
Regarding it globally, then I'd say yes. In more detail, each bus will power different systems. More important systems (like flight instruments) can be supplied by different power sources and/or busses.
Also, not that important stuff, like cabin lighting, the ovens etc are all connected to a single bus, (or two busses) and can be easily shed (disconnected) if you lose an engine or a generator.
Important stuff, or sstems that always need electrical power to function (ship's clock) are connected to busses that are always powered (as long as the battery has enough power). These include the fire extinguishing system, as an example. This would then be your hot battery bus (Boeing term, not sure if it applies to Airbus).
Regards,
Mark