How can I tell when my fat is sufficiently creamed?
The mixture should lighten in both color and texture, and it should be 'fluffy'. It should also increase in volume.

The point of creaming is to incorporate the sugar with the fat, while at the same time adding air to the mixture. The air bubbles introduced during creaming expand during cooking, making the cookies rise and giving a lighter texture.

Cookies baked with under-creamed butter would therefore remain fairly flat and dense.