People say this to another person to be polite when they don't agree with what they are saying.

Yes, when they no longer want to discuss the topic it is easier at times to say a phrase like this. It usually will end the conversation or changes the topic. It can also be said very sarcastically as well if someone is very upset or displeased (the say it with an attitude) .  Hope that helped! 

thanks <em>JURA </em>and <em>JOYCE. I think I got the point. </em>