In grammar we have what are called independent clauses and dependent clauses. An independent clause, in simplest terms, is a group of words that forms a valid sentence on its own. An example would be, "Giada threw the mascarpone cheese at Rachael." The sentence has a subject, a verb, and in this case, an object as well.
A dependent clause is a group of words which does not qualify on its own as a sentence. This is often the case when the phrase begins with a conjunction (connecting word) such as "because," or "although," or "when." For example, the phrase, "Because I am hungry" is not a sentence on its own. It does have a subject and verb, which is all a sentence needs, but the fact that it starts with a conjunction means that something else, namely an independent clause, must follow.
As another example, the phrase, "Although it was a nice day" is a dependent clause and not a sentence on its own, but it becomes a sentence when combined with the independent clause, "the park was quite empty." We combine the dependent and independent clauses with a comma.
To address the topic of the article, the following sentence is completely valid: "Because it was raining, I decided to take my umbrella." It follows the rule explained above. It is absolutely wrong to say that it is not a sentence just because it begins with the word "because."
Students and of course business writers should feel fully comfortable with this concept. Writing becomes more interesting when we utilize a combination of dependent and independent clauses in accordance with the rules of grammar. Because of that fact, we should try to do it!