Quote Originally Posted by Cumulus View Post
In other words, in this sentence, if one aims to apply the verb in an accurate way, one not only needs the accusative, but the instrumental as well
"Собой" is not required. It's optional provided that it is correct. I think we only can use "собой" if "покрывает" means that something lays on something else and covers it. In the example sentence it has a slightly different meaning. It means that something is spread over all area.