where do I use the Facebook API key and API Secret

Where do I use the facebook API key and API secret.

I seem to work around without them.Would that cause any issue? Even without them I am able to get my friends开发者_运维知识库 and POST to wall.

You only need them, when you try to access data, which is not publicly available. For example, if you choose to show your wall only to friends, but want to retrieve them in your app, you'll need the key and the secret (and the app-id).