I say that a lot when someone stridently believes something to be true, but has not convinced me of the fact. It is an affirmation of their belief to end the argument insofar as their conviction is obvious; but that does not mean what they say is true.
The statement is by no means meant to construe that I give any credence whatsoever to what they are saying; but that I am tired of listening to the person’s assertion of fact without proof. Essentially, it also means that I don’t (yet) completely discount what you are saying, but require more proof.
I am a programmer, so I get to say it a lot when someone says “this is why that code is broken” or “that code you wrote does not work.”
I also get to use it with my conservative friends and other people, like climate-change-deniers, who make wild assertions with no proof, like “global warming is just a conspiracy by the communist Chinese to get us to slow our economic development.” If that was their plan, all I can say is: too late– we lost economic supremacy when Walmart, the outlet for all unnecessary junk made in China became the one place to shop for rednecks.