As you know, there are two push buttons (MASTER SW and START) related to the APU in the Airbus A320 family.
Why there are two pushbuttons? I mean is there any condition in which users switch only one pushbutton on?
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Are there cases that the APU MASTER SW is used but not the Start PB?
Yes: diagnosis, emergency, and maintenance.
Turning on the APU master switch without pressing the start button triggers the following actions:
If something is not working, you can try to see in the ECAM page what is happening without actually starting the APU.
The procedure does not require to actually start the APU.
Many maintenance procedures require to power-up the APU but not start the APU. One case is also in the FCOM: