will be more immersive if there are some kind of reputation associated with the npc then they answer you more smart if exalted.
If you are developing an achivements system depending on which you have the relationship changes. For example, you has killed 70 PetaQuantumTrillions of a race, so the people of that race talk to yo with fear, or even they run away for a few meters.
Rude persons that if you are more than 10 seconds looking what their are selling close the selling windows and blame you.
Every time that you enter to a zone they follow you like a headless chicken trying to vendor you the most precious jewel from the rarest and dangeours mine of the most far away mountain opening you trade, after two times open, he fixes in another one until you come back after an internal cooldown.
some npcs that bring you chain quests, has a little dialog that gives you the possibility to get better or worst loot at the end of chain.