Pre statement declarations of honesty are only made by people who 1) aren't about to say something honest, or 2) are very unsure of what they're saying.
Trump says "to be honest with you" all the time, as an example. People are entitled to believe that a person they're listening to is speaking honestly. A person who repeatedly starts his comments with "to be honest with you" has a different world view. It implied the speaker normally lies, and is probably lying right now.
