"Passive voice" has a very clear definition: It's when the grammatical subject of the sentence is not the agent of the verb in the sentence. For example, "my car was broken into". The subject is "my car", but the verb is "broken" and the car isn't the actor who did the breaking. In fact, in this sentence, the agent of the verb is completely missing, which is one of the main reasons to use passive voice: when you don't know who or what the verb's agent is, and they're not the focus of meaning anyway - you don't *want* to mention them, because you're talking about your car (which got broken into), not the people who broke into it.
That's another common reason for passive voice: even when you do mention the agent of the verb, you deliberately want to put the focus of the sentence on something else. For example, "while visiting Al-Aqsa Mosque, King Abdullah was shot by a Palestinian assassin." If this sentence appears in the context of a brief history of Jordan, in a paragraph describing the life and rule of Jordan's first king, it makes sense to keep the sentence focused on him, and what happened to him; rearranging that sentence into active voice would make it fit in more awkwardly, because it would look like you're writing about the assassin rather than writing about the king.
Sometimes, though, passive voice is useful for the opposite reason: to give you a way to emphasize the agent in particular. For example, "Some people get sued by their competitors, but this guy was sued by his own law firm!" The first half of that is active voice, but the second half is passive voice in order to emphasize who sued "this guy" even though he's the subject of the clause (which makes it a better sentence, because it's a cleaner contrast with the subject of the other clause, "some people").
BTW, look at the T-sentences in this comment. One of them even begins with one of your examples, "that is", though I abbreviated it: "That's another common reason for the passive voice:". This is active voice, of course. Whatever contortions you might go through to remove the verb "to be" from that sentence would leave you with one that does not express the point that sentence is there for. I put the paragraph break there for a reason, but I also wanted the next paragraph to clearly refer to the end of the previous one and make a point about it.
I know that a lot of style guides go on about avoiding what they term "passive", often without a clear or consistent definition of "passive", but IMO they're giving awful advice that they generally don't even follow themselves. If you're driving down the road and see a big cat ahead, "there's a big cat in the road!" is much better than trying to figure out an agent to stuff into your sentence as a subject.
Re: Oh, let me clarify myself!
That's another common reason for passive voice: even when you do mention the agent of the verb, you deliberately want to put the focus of the sentence on something else. For example, "while visiting Al-Aqsa Mosque, King Abdullah was shot by a Palestinian assassin." If this sentence appears in the context of a brief history of Jordan, in a paragraph describing the life and rule of Jordan's first king, it makes sense to keep the sentence focused on him, and what happened to him; rearranging that sentence into active voice would make it fit in more awkwardly, because it would look like you're writing about the assassin rather than writing about the king.
Sometimes, though, passive voice is useful for the opposite reason: to give you a way to emphasize the agent in particular. For example, "Some people get sued by their competitors, but this guy was sued by his own law firm!" The first half of that is active voice, but the second half is passive voice in order to emphasize who sued "this guy" even though he's the subject of the clause (which makes it a better sentence, because it's a cleaner contrast with the subject of the other clause, "some people").
BTW, look at the T-sentences in this comment. One of them even begins with one of your examples, "that is", though I abbreviated it: "That's another common reason for the passive voice:". This is active voice, of course. Whatever contortions you might go through to remove the verb "to be" from that sentence would leave you with one that does not express the point that sentence is there for. I put the paragraph break there for a reason, but I also wanted the next paragraph to clearly refer to the end of the previous one and make a point about it.
I know that a lot of style guides go on about avoiding what they term "passive", often without a clear or consistent definition of "passive", but IMO they're giving awful advice that they generally don't even follow themselves. If you're driving down the road and see a big cat ahead, "there's a big cat in the road!" is much better than trying to figure out an agent to stuff into your sentence as a subject.