in some languages and cultures (especially yiddish), certain expressions are understood to be sarcastic. I think 'I could care less' was originally used in this sarcastic tone (akin to 'Tell me about it' in US circles)
It's just a pointless statement that has been echoed enough times that we've become accustom to the sound it makes, much like a parrot that mimics 'Polly wants a cracker'.