Why do the Wyld Pacts glow after a matching Smite?

[smoky bear]

I know a matching Smite for the weapons increases the damage of the next heavy attack, and Paragon can stack that up to three if you get one of each color, but what does that do for the Pacts?
I thought it'd be empowering the next ability cast, but even a matching color ability doesn't consume that glow on the Pact, so I'm kinda lost on what it does.