新概念14课里面的练习(关于过去完成时)有个疑问

新概念14课里面的练习(关于过去完成时)有个疑问
题目是句子改写:
(after)she wrote the letter.she went to the post office.
把两句改为一句.
答案是after she had written the letter,she went to the post office.
因为这课的语法是过去完成时,但是为什么不可以改成:
after she wrote the letter,she went to the post office.
这样可以吗,我觉得不一定要用had done啊!