Through the conversation, you learn the idiosyncrasies of the other person. In a culture where haggling is normal, their “style” gives you insight into their personality and how they might act in other scenarios. And it identifies you as an insider or outsider to the culture. At the very least, you’ve learned their name and face.

I also hate haggling, and since I’m not from a culture where it’s a norm I can’t use it to read deeper into a person. (Or them into me.) But it’s not too hard to imagine how it helps to build relationships.