What's the purpose of a parchment lid?

http://remcooks.com/2012/10/19/how-to-make-a-parchment-paper-lid-parchment-cartouche/

A parchment paper lid is another one of those French techniques that has been around a while. In France, it is referred to as a cartouche. Its purpose is to control the rate of evaporation, which in turn slows the reduction of moisture and concentrates a sauce or stew in a much more efficient way. It’s used in preparations where you are trying to control heat and evaporation, for instance poached fruits, onion confit, braises, etc. In these applications, if you simply use a standard pot cover the heat inside the pot will get too high resulting in the food cooking too quickly, turning to mush and not holding as much flavor. On the other hand, if you leave them uncovered, too much moisture will be lost and the food will not be done. Another effect would be the food will also be exposed to the air resulting in it becoming discolored.

The problem with lids is in how they drip -- the water tends to condense and run down the sides, making it so there's uneven moisture in the dish. Some dutch oven lids solve this by putting little nubs all over the lid, so it drips more evenly all over the dish. With a paper lid, you don't get that large collection of steam in the air above the dish, so you don't run into those problems. You also won't get as much total evaporation, as there's less liquid-to-air surface for the liquid to evaporate from.

You could go with a just a skewed lid, but it's possible that the rice might come out less than ideal. (although, I'll one for taking shortcuts ... if I can get an 8/10 with 1/2 the effort, I'm all for it -- you just have to know how the shortcut might change things ... this one's a bit of a toss-up, because using a normal lid means there's one more thing to clean, but that's balanced by the extra trash from the parchment lid).

For my 2 cents, I've never ever used a parchment lid in recipes that call for it. I think Joe's explanation of the purported purpose is right, but I just can't see it making a whole lot of difference, and I've never noticed any moisture problems in the dishes I've made this way. I'd love to hear from someone who says they can really see a difference having made a dish both ways.

I'd like to know where this recipe originates from...my guess is from a chef or restaurant based cookbook. Parchment lids are generally used in restaurant kitchens for various reasons, but generally they are disposable and can fit onto any size pot when needed, thus eliminating the need for lots of lids. This is cheaper in the long run for the kitchen as well, and parchment takes up less space. Plus for some cooking methods like poaching, nothing beats a parchment lid to keep delicate items submerged in their cooking liquid.

It could also be that the author found the parchment lid as a superior method for keeping all of the ingredients submerged in the liquid while simmering? I bet that a partially covered lid would do the same job, but it is hard to say without having the entire recipe with method to examine.