Let's look at a very famous active sentence.
Shakespeare wrote Hamlet.
Now let's put that into the passive.
Hamlet was written by Shakespeare.
So why do we use the 'by Shakespeare'. Well, read the sentence without it and it makes no sense. Also, ask yourself why we would have used that sentence in the first place.
Teacher: Who wrote Hamlet, class?
Freddy: Bacon, miss.
Teacher: Nice try Freddy, but Hamlet was written by Shakespeare.
Often though there is no 'by' clause.
John: Why so glum, matey.
Fred: My car was stole last night.
Fred wouldn't say 'My car was stolen by someone' because 'by someone' sounds silly.
So, in answer to your question, the by phrase is used in the passive voice when it is needed. Whether it is used (by people) more frequently, I don't know. (Did you see what I did in the last sentence?)
You do sign-language if you have no voice.
Bill Engvall is the comedian who coined the phrase, "Here's your sign".
No, "to sign" is the infinitive form of a verb.
Yes, it is classified as a 'passive' device.
To sign "voice" in American Sign Language, make a "V" hand shape near your throat and move it forward slightly.
The first noticeable sign of the male voice breaking would be it cracking on some words or during telling/screaming.
A sign of puberty.
There was a sign of monotony in her voice.
subject
Chemical equations have frequently an equal sign (=) between reactants and products.
This is a sign that you are done with puberty.
an expression. or an inequality.