We usually use this when we are compelled to agree, or we feel like in special circumstances, we must agree. It almost carries a little bit of a hesitant tone in English, in conversation. "I would have to agree with you. Bobby's pizza really is the best in town." [the implication is that you WANT to like Larry's pizza, perhaps because you are his friend, but Bobby's pizza is by far a superior pie] I know that's a complicated answer but I hope it helps :)