Because they aren't familiar with or enthusiastic about the code I give them, they could be about code they have written. It's the classic "describe one time you did something poorly/well"-question, but with code. I want to see them critique some code, or explain what's so great, or why it ended up the way it is.
Reasoning about a problem i have (even showing some code) is also a good part of an interview. But it's my side of the field. I just found it's much better to move the interview to the interviewee's side of the turf, because they are more comfortable there.
