The carrier is really a mylar sheet which has a low-tack adhesive coating which enables you to do three things.
Firstly, if you are using a smaller than A4 piece of media, you can use it to affix the media whilst cutting - which makes it ideal if you just want to rev up your cutter for a small logo or something without wasting a whole sheet of material
Secondly, if you are using a material that you intend to cut through (or which doesn't have a backing paper), it keeps the material in place whilst you are cutting.
Thirdly, it helps prevent you from damaging something that's called the cutting strip. This is something that is situated on the cutter and which gives the cutter a consistent and flat surface with which to press down upon. If you mis-set your cutting blade depth / force, you can cut through your media and backing paper and scratch the surface of this cutting strip. The strip is replaceable (on the C330-20 anyway) but even so, it's clearly best to try not to damage your cutter.
With my C330-20, I always use the carrier sheet. Over time, the adhesive loses its tack but I bought a spare pack (2 pieces) and I intend to try and refresh the old one with some spray mount or something similar.
I can't speak for the Lite but the C330-20 gets a massive thumbs up from me - it's ace.
It's really one of those purchases that is always difficult to justify for casual use but once you do, you'll never look back and you'll keep asking yourself how it was that you ever lived without it.
If you've not found them yet, click
here for a decent supplier. I get my vinyl from these guys and my CR consumables. The deliveries are super and even when I ordered the wrong carrier, they let me send it back and exchange it for the right one. Can't say fairer than that. Also have to say that the CR 330-20 Cameo (new model) looks the mutts nuts.