Why is the `touch-action` CSS property needed?

Perhaps I've overlooked something but I haven't found anything in the
Pointer Events spec nor mailing list archives that explains why the
`touch-action` CSS property is needed.

>From the outside perspective of someone who is *relatively* new to Pointer
Events, it seems to me that the JS/DOM should always receive Pointer events
and that any user agents' special touch actions should be triggered as the
default actions for a given Pointer event.  This would mean that you could
prevent the user agents' special touch actions simply by calling
`e.preventDefault();` from the JS side.

However, what I've read so far about this `touch-action` CSS property would
suggest that the user agents may trigger their special touch actions *
BEFORE *the Pointer event is passed to JS/DOM.

I'm also confused why we would want to *require* a CSS property in order to
*enable* a DOM event (though I do understand the nicety of using CSS
properties to *disable* DOM events, e.g. "user-select:none").

Could someone please enlighten me?  Thank you in advance!

Sincerely,
    James M. Greene
    http://jamesgreene.net/