As noted by some others, such a sign would help deal with the mistaken "hey look they left and forgot their stuff" situation, which we have encountered several times. For some years now I just print off a "campsite occupied" sign, stick it in a large Zip lock bag and attached it the picnic table with the table cloth clips. Haven't had any issues since.
We've also had our assigned sites at campgrounds poached several times. Our little sign and a cheap table cloth seem to have taken care of that issue too (people need to have some visible indication the site is not vacant and, as noted earlier, the little placard on the post etc doesn't always do the trick. Yes, you can get them moved but it's a hassle), although I'm sure determined poachers may try no matter what.
Of course, "real" thieves don't care. I will leave our electric cable and water hose at some campgrounds but, if I'm not comfortable with the surroundings, I take the time to roll them up too and take them with us. So far so good. Being "travelers" and not "campers" we don't have much stuff out anyway.