Necessary? Not really. If your abs and back are up to snuff, it won't make much of a difference. Fred Hatfield squatted 1014 with his belt uncinched at a loose setting.
That being said, wearing a reasonably loose belt isn't going to hurt your core strength, and will provide some support if you start to lose form, will keep your back warm, and give you some form cues.
So it's a good thing, but not necessary if your abs and back are strong.