Noob Cross-up Question

Why do players use cross-ups when the opponent is stunned? Specifically, Firestorm players. I was watching the Northern Arena tournament and noticed Firestorm players would do it after a meter burn Molten Trap. They obviously have time to just to into a combo, so what's the advantage they get with the cross-up?

Combo damage scales so instead of extending the combo you go for a reset, resetting the scaling for potential more Dame. Basically the law of diminishing returns. For the amount I'm put in you dont get as much if you continue. Also firestorm has a vortex.

Thank you.