。
She didn't need to bring a bento.
Literal
She [topic-は] bento [object-を] bring need [contrast-は] didn't-exist [despite-のに].
~必要はなかった means 'there was no need to.' The sentence-final のに adds a tone of regret or reproach — 'she didn't need to, and yet (she did).' 弁当 is the Japanese packed lunch, a cultural staple.