Why is important that scout is wearing her Halloween costume during the attack by bob Ewell?

It is important because her costume, a ham costume, ultimately helps to protect Scout from being harmed during the attack. The costume acted as a cushion that absorbed the brunt of the attack, making it easier for Boo Radley to intervene and save Scout from further harm. Ultimately, the costume serves as a symbolic element that highlights Boo Radley's unseen presence and protection over Scout.