It is literally of zero importance, to the point of being a dump stat.

PE lets you scroll offscreen a little further with each point. But throwing rarely (almost never) targets anything offscreen. So unless your build (gunslinger, trap spotter, etc.) requires it, don't waste any points on it.

As slamdunk said, DX is what increases your throws per turn (in turn-based) or throwing speed (in real time); and also qualifies you for higher ranks of Throwing, which increases your overall accuracy.

Edited to add: One of my favorite backgrounds is the Professional Knife Tosser; at the cost of one point of PE, you get what amounts to almost a full rank boost to your Throwing Skill. This lets you get both expertise and mastery that much sooner; although you'll eventually want to max out your DX anyway, for the speed bonus.