The phrase "to be honest" is often used as a filler which makes a sentence run on. It is often perceived that one who uses such phrase implies that they often are not honest, so they feel the need to say "to be honest" when they actually are being honest.
Filler phrase so you can quick come up with a random statement. Often followed by a very unthoughtful statement that the person did not in fact mean to say.
Other ways it can be used is if you are in fact trying to be honest.
1. To be completely honest, I really think you complain too much... Wait I didn't phrase that right!
2. Gavin-To be completely honest, saltwater is actually okay to drink large amounts of.