don't have to 和 needn't 都有“没必要”的意思，二者有以下几点区别：
从搭配上看，don't have to do sth, 如：You don't have to pay at once.
而needn't do sth, 如：You needn't pay at once. 此时，need作为情态动词，所以之后接动词原型
再补充一下，当need作为普通动词时，和作为情态动词的用法搭配是不一样的。如：You don't need to pay at once.
从使用频率上看，don't have to比较常用
从意义上看，区别很轻微，举两个例句来说明一下don't have to和needn't的区别：
You don't have to do that. 偏重于说“不要担心，你没必要做那个”。意思是你没有做那件事的责任，所以没必要做。
You needn't do that. 偏重于说“不要认为你不得不做那个”，意思是这件事不重要，所以没必要做