General Cooking Questions

Hi Chefs,

So I went through the Stella blog and listened to the podcast on poaching and braising.

However in a poaching fish fillet recipe that I followed and cooked. It says to cover it with buttered aluminum or parchment paper then bring to poaching temperature.

I would like to ask what is the explanation about applying the butter in the aluminum/paper? If I cover it with its lid do I also always need to apply butter on the underside of the cover lid when poaching meats?