Could be for the purposes of being able to identify them if they do call/message/email you (even if you don't contact them yourself)? Maybe for the purposes of blocking them (in iOS 7)? Maybe just to keep the contact information for a "just in case" type of situation.You apparently installed the rekindle app.
Seriously, if you never contact a contact, are they really a contact? But I'd agree with the others suggesting you accidentally selected it at some point. No biggie.