Past necessity

Needn't have + past participle
This structure is used to talk about an action which was unnecessary. It indicates that the subject performed the action.
I needn't have preparated so much food for the party; everyone had eaten before they came. (I prepared lots of food but I realized afterwards that it wasn't necessary)

Didn't need to + infinitive
This structure is used to talk about an action which was unnecessary. It usually indicates that the subject did not perform the action.
I didn't need to prepare very much food for the party; everyone said they would bring something to eat. (I didn't prepare lots of food as I knew it wasn't necessary)