Necessity is the mother of invention, or so I've been told. About ten years ago I started putting Reynolds Oven Bags in my crock pot before cooking anything mainly because I was tired of cleaning up the mess and I loved the ease of throwing away the cooking bag for other meals and thought it would work for the crock pot and it did. The second useful reason I used the cooking bag in my crock pot was because I could be assured that the ingredients would stay in the pot when I tied it up- rather than sloshing out. I realize that many cooks now have the newer versions of slow cookers that have fixed the problem of transporting meals- but if you don't, you might want to try a large size bag to keep your stew inside the pot rather than on your car carpet!
I wasn't surprised when Reynolds came out with their own version of Slow Cooker Liners. I still choose to use the bags, maybe it's just an old habit, or it might be because I pinch pennies and there are five bags instead of four in a package (40 cents each). Or, it might be the aversion to scrubbing.