Drop a large vise or rotary table etc you're probably doing damage either way so what all do you protect from
Oh, you would be surprised. It isn't dropping the rotary table or vise. You are right, it ain't gunna protect against that. But a flying broken end mill, a dropped collet, a dropped TBolt, pair of pliers, can of whatever. It doesn't take long to accumulate battle scars. I accept that it will never stay flawless, but I want to reduce the damage with reasonable preventive measures. And I think that a piece of heavy corrugated plastic is the perfect answer.
Not long ago, I was discussing corrosion with a guy who visits machine shops and I suggested plywood. He instantly said it wasn't that good a solution because wood products harbor chemicals that can do as much damage as rust. He recommended corrugated plastic. I use cardboard now, but I'll switch to currugated plastic as soon as I can find some.