[Richard G.]

Presumably just to keep wet and dry things apart?

I use dry bags for my down stuff (sleeping bag and smock), since they also compress (air-exchange thingies). But I still put the tent loosely in a plastic bag, partly as belt and braces to keep the water away from my dry stuff, but I tend to leave the tent uncompressed because it's last thing in the pack and I let it fill up the remaining pack space for comfort/stability. Never thought it was a good idea to compress a tent either though.