You can press the home button until the phone plays the siri tone, and then talk, and wait for siri to detect end of speech. Or, you can press and hold until you hear the tone, and continue to hold as you speak. When you release the home button, end of speech will be triggered. Instead of having to wait. This can be useful in places with ambient noise that prevent the end of speech event from happening automatically. Previously this was remedied by tapping the microphone icon on screen to signify end of speech.

I don't see the point of that either. My home button is going to suffer a lot, and it's already unsteady.

It's optional. You either release the button and it acts normally, or keep pressing and when you release it stops listening.

The headline is pretty explicit: "siri knows when to stop listening".

Unless it's a change necessary for the CarPlay stuff.