Again thank you for your responses. It seems you all know the system schematics, but my question may have been written unclearly.
Still my question stands: Why does the 737-800 have a battery bus and a switched hot battery bus?
The only situation where just one of the two is energized is (as stated by several of you) when the battery switch is off and the standby power switch is set to bat. But when would one do this?
I myself cannot think about any situation.
So in all normal cases the battery bus and the switched hot battery bus are powered at the exact same time (i.e. when the battery switch is on).
(Even though the battery bus is energized via TR3 when the transfer buses are energized, it turns off when the battery switch is set to off).