This question already has an answer here:
I've read this: What can an app do with the "USE ACCOUNTS ON THE DEVICE" permission??
In the main answer (by Dan) it's said:
The AccountManager then asks you if you want the requesting app (YouTube) to be able to use the requested account (Google account). This might be in a dialog that appears over the app, or in a notification.
However, upon installing something, e.g. Facebook (which adds facebook account as I understand) and starting to use it, I don't remember some system message asking me to grant access to facebook account to facebook app. Facebook app can post on facebook w/out using facebook account?