Independent clauses can stand on their own, even if they are joined together in one sentence. Subordinate clauses, on the other hand, aren’t supposed to stand on their own. Because they depend on another clause in the sentence, an independent clause. That last sentence, beginning with because, was a subordinate clause that I forced to stand on its own. It would have fallen flat on its face if you hadn’t automatically connected it to the sentence before it.
The word because is an example of the kind of word that often introduces subordinate clauses. The word because answers a question and your reader has to know what the question is, or it won’t make sense.
In informal writing, in conversational writing, you can often get away with putting a period after a subordinate clause, even though it isn’t technically a sentence by itself. In fact, it’s often a good thing to do simply because it makes it sound conversational, as long as our reader understands what the subordinate clause is referring to. In formal writing, however, don’t put a period after anything but a sentence.