This one is simple. If, during the early stages of the game, I talk to a character on my own free will and get rewarded with something worthwhile, I’m much more likely to talk to other characters looking for more handouts. A steady stream of dialogue rewards enforces this likelihood throughout the entire game.
If you’ve spent a good deal of time on your writing and truly believe your characters are saying things that your player would enjoy hearing (because you’re funny), this is a good way to motivate a generation of jaded gamers into giving your dialogue a chance. Since the reward is instant, players are already happy. And at that moment you have their attention, so if you’re going to sell your dialogue this is the time.
Please note that if your dialogue is not up to snuff and you offer random rewards, players will still talk to everyone and they will mash their controller buttons to skip the reading and get right to the presents. We don’t care that the princess is in another castle. If Toad’s got a million dollars, we still don’t care that the princess is in another castle; but thanks for the million bucks. If I’m an assassin in the middle of a banquet, and some cordial fellow with a bowl full of laughs at the entrance gives me poison daggers, I will be motivated to talk to other people at the party and go on to complete my objectives in a timely manner.
And believe me, if you don’t motivate my assassin character and give your NPC’s a reason to live, I’ll probably spend the next hour trying to figure out how to murder everyone at the banquet without getting caught. The only talking I’ll do is to stop and talk to the last guest standing to see if they say something banal like “I’ve been looking forward to this banquet for three months” instead of what he should really be saying, which is “Dear god why am I waist high in blood”.