The containment field kept the Fiendfyre from escaping the building. Without it, the unpredictable and dangerous spell could potentially take over all of Diagon Alley. She knew it was a risk when she cast the spell. Going down in history as the witch who singlehandedly destroyed Diagon Alley with a spell might've been amusing, but she was glad that she hadn't completely destroyed the iconic location. She'd had some wonderful memories there.