1. A sentence must have a subject and a verb if it is to make sense.
Incorrect: John, being a friendly computer salesman and baseball fan.
Correct: John, being a friendly computer salesman and baseball fan, refused to argue.
(John--the subject--is doing something, namely, refusing.)
2. A subordinate clause (also sometimes called a dependent clause) is not a complete sentence if it does not have a main clause even though it may have a subject and verb.
Incorrect: Because we are baseball fans.
Correct: We watched the All-Star Game because we are baseball fans.
There is nothing wrong with beginning a sentence with the word because as long as the clause with because is followed by a main clause.
Correct: Because we are baseball fans, we watched the All-Star Game.
3. Sometimes in conversation only sentence fragments make sense.
OK, if you are recording a conversation, otherwise incorrect: She asked, "Why did you watch that baseball game?"
"Because we are baseball fans."