Why does the the actor root component affect physics/collison?

I have a fireball actor, it’s root was the default scene root with a sphere collision comp, with a particle system and an actor projectile but I couldn’t get it to behave the way I wanted which was be shot, have momentum and react to collision. But changing my root from the default scene to the collision fixed it. My question is why? Is it that the sphere being the root makes it do what I want? Or is it the fact that the default scene root was giving some kind of properties to the actor?

Second question, with actors with many comps what should I try and make the roots? What works best as the root? What shouldn’t I make the root?