The use of a comma before because is best seen with negative statements.
I didn't run because I was afraid. (I ran because I was late.)
Without a comma, this means that you did run, and that you ran for some reason other than fear—as explained in the sentence in parentheses that follows it.
I didn't run, because I was afraid. (I was too scared to run.)
With a comma, this means that you didn't run—and it was fear that prevented you from running.
With positive statements, the comma doesn't change the meaning.
I ran because I was afraid.
I ran, because I was afraid.
Both of those sentences mean the same thing. However, the comma doesn't do anything syntactically. In terms of a concise statement, the lack of a comma is better.
However, some people may add one for stylistic effect. Although, if writing for stylistic effect (where liberties can be taken with grammar), it would probably be considered more appropriate to use a dash or even break it into a second sentence.
I ran—because I was afraid.
I ran. Because I was afraid.
Alternatively, a comma can be used if it's a conjunction that immediately follows it rather than because itself:
I ran, but not because I was afraid.
I ran, but only because I was late.