Same as in English, we have to be careful about saying 'I know' sometimes even when you 'knew already' you should not always readily say so - so I think 明白了 is a way to be polite, even though it is not strictly correct. (其实已经知道了，但是还该说‘明白了’ 　To be polite in English you would say 'ah, I see' whilst under your breath muttering 'come on, I always knew that!'

明白了=understood

We don't express like that in China,we will say 我早就知道了 or 我知道啊。

只說「明白了」語感上是在A說明後，才理解的狀況下回答的。 可以用「我早就知道了」、「我早就明白了」來強調原本就知道。

“明白了”，we can think it as , the secret(knowlegde) came to light for me. If someone told u a interesting information, you can answer "哦，是吗？"（Oh，is it？），otherwise “嗯（哦）（知道了）”.