If you are a king, you can hand it to a count vassal for a big opinion boost without them becoming independent. It's titular, so they aren't going to start desiring counties belonging to you or other vassals either. I don't think it gives them any more power either - they still have the same amount of troops. It does bring their ransom value and released prisoner opinion modifiers up a notch, which can be useful if you are going to have perennial troubles with a vassal and their heirs, but you don't want to revoke their titles for some reason (I have been landing dynasty members in my current game and my cousin is relentlessly factioning. About twice a generation I provoke the revolt, then get a +15 opinion for crushing the revolt and another +9 for releasing him (or take the gold ransom if that is more worthwhile).
If you hang on to the title, you do get more prestige per month, however. So if you can manage the - 10 opinion then there's no reason to get rid of it.
Why do people destroy these titles?