It's like a small warning to make someone prepared for something. "Just so you know, the boss is in a terrible mood. You probably don't want to talk to him about your vacation right now."

"Just so you know, Bob can't be trusted with personal information." Telling you something well, for your own knowledge. It is used when you are giving information to another person so they will know something - even though they didn't ask. The same as "For Your Information', and that what they are about to tell you will, most of the time, be useful. Example: Just so you know, there aren't any service stations along the road just so you know, it means just so you know. its also a patronizing term to me. I think it may be a psychological attention getter. "just to tell you" or "for your information" Telling you something whether you want to know or not. They are going to tell you anyway.