A December 14, 2017 application from Apple to the US patent Office shows Apple’s interest to patent, “a digital assistant that is capable of detecting a whispered speech input and providing a whispered speech response.”
Why would you want Siri to whisper to you?
One benefit could be to have a digital whisper bot in your pocket for ASMR sessions. Siri would not be the first virtual ASMR artist though, Amazon’s Alexa can already whisper (see link at bottom).
I don’t think ASMR sessions are Apple’s intent. Their proposed benefit is more about privacy and courtesy.