Do you mean the man that sits in the Market as a child, and in Kakariko as an adult? He's simply there for when you are short on rupees. He's not really important to the rest of the game that I am aware of. What he means, though, is that you can give him the content of bottles in exchange for rupees. I personally, am never really in need of rupees after a certain point in the game, so he isn't really too much use to me. I suppose if you always get your tunics eaten by like-likes or are just always purchasing stuff, I guess he might be more useful to you.
So, you just wanted to know his purpose, right? That's pretty much it.