It’s fair to say that you can fry in the bottom of a billy can but it’s awkward to say the least. A dedicated frying pan is well worth the extra weight for most people.
Try to get something that has a reasonably thick base that will spread the heat because the temperature differential between the heated centre and the edges of the pan exposed to cold air can easily lead to scorched food in the middle and uncooked stuff at the edges. Not good where meat is concerned. If the lid of your billy can also fits your frying pan that is a very useful bonus as that will stop the surface of the food not in contact with the pan from cooling quickly in the cold air. A biscuit tin lid may make a good substitute or even a piece of aluminium and a bit of work with a hammer will do. (punch a hole and make a string handle so that you can lift it.)
Might seem a surprising one this but if you add a pierced plate or or grill sheet its possible to steam or even steam bake in a billy with a lid and some water in the bottom.
You will need something to keep the grill plate raised above the water but at a pinch, three pebbles can do that for you.
A tricky thing to get right with a stove normally but a wood or charcoal burning stove can be used for grilling food just like a fire can. Most liquid fuels tend to taint the food though.
Another thing that a lid is useful for is storage. It is all too easy to misplace small items such as cutlery in a snowy environment.
You might leave something in a spot where you know it is only to discover that it is covered with fresh snow in the morning.
Collecting bit and pieces in your cooking pot and putting the lid on is a good way to keep everything together.
A cooking pot can also be used to keep water unfrozen overnight by burying it in the snow, which is an excellent insulator.