Because it can establish a lower bound. When we say "we'll never know the real number", we are saying "the real number is at least X, but it could be a lot more

Because it can establish a lower bound. When we say "we'll never know the real number", we are saying "the real number is at least X, but it could be a lot more". If X is already high in our eyes, it can be useful for providing a view on the true magnitude of the problem.