It's a social ritual thing. Introducing yourself by way of a gift, or meeting with an exchange of gifts, has been a tradition across human culture for millenia. Of course, if the gift is useful, it's best, but what is more fundamental is the gesture. It's saying, "Hey, welcome! I'm a friend and not a threat. To demonstrate that, I'm going to freely give you some of what I have."

So if one's neighbor brings them something they can't eat, they just thank them and decline, or ritually accept it and then quietly dispose of it.