The diagram of the keyless unit I have installed shows that it should be connected to the ACC wire. Please see below.

Anyone know the purpose of that connection since the unit already has constant power via the +12v connector?

 

the alarm module requires +12VDC ignition/ACC power for it to work properly. Just about all car alarms require that power source.

 

Understood that it may be needed for proper operation, but my question is why that should be the case when the unit is already connected to constant 12VDC... BTW, I have connected this particular unit without the ACC connection and it is working just fine.

In a car alarm system, I can see the ACC being useful because it could be used to prevent you from turning on the alarm when the car is ON. But what is the purpose of this in a simple (non-alarm) keyless entry system? To prevent from locking/unlocking the doors when the ignition is ON? That is a bit non-sensical.

 

ACC/ignition or switched 12VDC power turns on/off certain features of the system (alarm or keyless). On the control board, it requires it. On Hondas/Acuras, finding the appropriate wires is super easy.