What does the candy do?
Solution 1:
So far I have found one use: to get answers to specific questions from your friends.
Eventually you'll have a lot of friends and a lot of answers. Unless you just want to sit around as read them all in the order your Mii presents, you'll have some unheard ones waiting. If you want to hear a specific one, or bribe them to tell you more answers, it costs 2 candy to bribe them. Both actions are options when visiting a friend.
That's all I've found so far.