Because I prefer it. It instantly loosen between sets, instantly tightens for the next set. No need to wrap the belt around a rack to pull it tight. I've been using one for the last three years, and so far there is zero downside. But if you prefer to struggle with a buckle, by all means do.
I've never needed a screw driver and don't carry one in my gym bag. But then, I have enough self-control not to stuff myself with Oreos.