[Enhancements] Question on Aura of Power

[Figleaf23]

The text says on p.4:

For a beneficial Affliction, the user may roll Will (as a free action) when someone he wants to bless enters his area of effect.

My question is, why is an additional roll necessary to affect someone entering a field that has already been established?

Would an activation roll be needed if a person entered an Area Effect burning attack?