I've got a friend who does this a lot. Apparently she uses it when she feels she needs to inform someone of something insulting, so she sticks the "no offense, but..." at the beginning. In some minds, I suppose, it indicates the speaker isn't purposely trying to insult the other, but let them know about an annoying personality trait or that they do look stupid. Before someone else actually insults them about it, I suppose...

LesiSputnik