“For good measure” is different than “just in case.”

For good measure means, “something probably unnecessary that’s said or done that’s in addition to what has already been done.” This is usually something gratuitous (unnecessary) added toward the end of something. So in the dispute between the US and China, USA accuses China of using a balloon for spying. China launches a full defense saying that the USA is over-reacting; the balloon was used only for peaceful purposes; it’s private property, not government property; it was innocently blown off course; shooting down the balloon was an unnecessary provocation. That was their defense. And then, in addition to that full defense, they said one more thing for good measure: you guys send balloons over here to spy, too.

Yeah, it’s weird because all countries spy on each other; even among allies. But there are unwritten rules and conventions. And to a certain extent everyone is play-acting their outrage when any one country’s activity is exposed because they know full well that everyone does it to everyone else. Still, I feel like if you get caught red-handed, you’re not getting the equipment you used back!

You can use “blown off course” in a figurative sense, but it’s not very common and it’s a little difficult. For example, you can say that “The negotiations got blown off course when one side introduced new, unrelated demands late in the process.” Or, “The project started off well, but was blown completely off course when two of the team members stopped attending the progress update calls.” Something like that. You could say it when you’re in a process that’s got a relatively defined path forward (project, negotiations, stuff like that) and something happens to make the process go in a much different, unwanted direction.

This is a good example, but “blow off” acts like a phrasal verb here and you’ll want to put the object (his boat) in between the verb and the preposition: “A big storm blew his boat off course and he was lost for a few days.”