When do you use "you don't say"?

you don't say

My doubt is whether you use this for exclamations or for agreements, all the references pointed out for saying it when one is surprised but I am sure I have heard some one say this in an agreeing way

I failed in my maths exam

You don't say!

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The weather is nice today

You don't say

It is down to intonation.

"They play horribly" "you don't say". - disillusioned and agreeing

Wiktionary:

• They're going to have a baby? You don't say! - surprised

• "Hey, guess what? There's a country called Brazil!" "You don't say." - sarcastic - the statement is obvious to the person uttering the expression.

You don't say is a response that can be used, when someone states something that is insanely obvious, and you cannot think of any other legitimate response to say.