It is known结果是为了让句子不要头重脚轻
【That knowledge is power is known to all,这就是个头重脚轻的句子】
It is known to all that knowledge is power.
Knowledge is power.这儿断句了,That is known to all.说的是那是人尽皆知的.
我们来看一个例句：
This medicine has some side effects.
By saying that/That is to say,it might be harmful to your health
英语国家的人不会说By saying this或者是this is to say,这都是中文式思维的表达方式.
外国人会指之前的事（即便是很近说的）为that,这是语言习惯.
单纯用中国思维的语法是搞不懂的.