If we want to ask for information we usually use the standard question form. However, sometimes we just want to keep a conversation going, or confirm information. In this case, question tags are often used to solicit input or confirmation...
Source: About English as 2nd Language
English as 2nd Language
I'm not a teacher, so please consider any advice I give in that context.
- Alan: You understand, don't you?
Bob: Yes, I understand.