She said that she would leave Beijing next month为什么要加个that,起什么作用呀,在什么情况下用,请具体说明,不用行不行

这是一个宾语从句，一共有两个句子，一个是主句“She said”“她说”，另一个是从句“she would leave Beijing next month”她下一个月要去北京，从句是陈述句时要在主句和从句之间加一个that连接

连接词 conj.
1.(引导名词从句)
He expressed the hope that we would keep in touch with his firm.
他表示希望我们与他的公司保持联系。
My idea is that we should act on his suggestion.
我的意见是我们应当按照他的建议去办。
That she is still alive is a relief.
她还活着,这是令人感到宽慰的。
2.(引导状语从句,表示原因或理由)因为,由于
We are glad that we have accomplished another arduous task.
我们都感到高兴因为又完成了一项艰巨的任务。
3.(引导状语从句,表示目的或结果)为了,以至于
He is working hard that he can catch up with the class.
他努力用功,以便赶上班上同学。
4.(引导表示愿望,感叹的从句,主句常省略)
That I could stay at home today!
今天要是我能呆在家里多好呀!

She said that she would leave Beijing next month.她说她在下个月起程去北京这是个复合句：主句：She said,宾语从句：She would leave Beijing next month,that是连接宾语从句的从属连词,口语中可以省略.that引导...

因为这是个said后面的宾语从句，that仅仅做连接作用，无任何意义，而且完全可以省略。
所以She said (that) she would leave Beijing next month.