In real life, as opposed to the isolated sentences you read in grammar textbooks, that "additional information" is always present. It is always part of the conversation, or the discussion.
That information is "necessary" in the sense that it provides the essential context for understanding what you mean. Meaning only occurs in contexts: what you mean depends on what situation you are responding to, who you are talking to, what information you share with your hearers.
And context to a very large degree determines how you say things, what words and grammatical forms you choose. This is why on ELL we constantly, ten or fifteen times a day, ask our questioners for more context. When somebody asks "Should I use present perfect or simple past in this sentence?" we have no way of answering unless they can give us the context: what is said before the sentence and what will come after it.
That necessary additional information does not have to be included in every sentence that refers to it. If the information has already been mentioned or is already known to your hearers, you don't have to mention it again, unless there is a possible uncertainty or you think they may have forgotten it. So all of these are perfectly acceptable sentences:
I was bathing.
The train had left.
I had been bathing for two hours.
But what you mean will not be evident unless the "additional information" is provided somewhere in the discourse.