Good question. There's only one reason: It is better than nothing.

In a "pocket" sized survival kit there really isn't room for a metal container. Traditionally, aluminum foil is included to improvise a cooking container. However, the wear and tear of carrying it around tends to cause pinhole leaks at the corners where it was folded.

This was my attempt to find something a little better for a pocket kit.

I'd never purposely go into the woods with just a pocket kit. I always bring a metal cup, among other things. But it is possible that you might end up with only what's in your pockets.