I didn't have it fitted on the vRS so it clearly isn't for heat protection, and after I did fit it I wish I hadn't as you can't hear the intake as well......
I have it on the diseasel GT as well, and it makes a lot of difference to the engine noise, it sounds like a tractor with the bonnet up. Tbh it sounds like a tractor with the bonnet down, just one parked a bit further down the road!