Yes, I agree with Christy in her comments. You have to include the phrase "for this" to convey the fact that what you are about to hear is going to be shocking, surprising, unexpected! Without the addition of "for this", it does not convey the same power. Examples: The husband comes in the door, the wife says "You may want to (or you better) sit down for this---I am pregnant! We are going to have a baby!!"

For this - is part of the phrase, without it, the sentence would be less idiomatic, and more literal.

@Christy Thank you so much for you comment. Why don't you just make yours an answer for my question? I would pick yours as the best answer...

It would depend on context. "You may want to sit down for this." This phrase means that what you're about to say is going to be so shocking that the person needs to sit so that he doesn't fall over! Now. You can say, "You may want to sit down," leads me to the same conclusion depending on context. Or, it might mean that I'm needing to sit down because of a long wait in a doctor's office, for example.