I do a lot of training for our new hire data analysts. My favorite lesson is explaining to them how this applies in the tools we use. You can get a feel for how someone's brain works by their reaction. Horror, wonder, or pop...always fun.
It depends on the context. What was described as NA in that example by be NULL or NaN in another application. For example, excel power query would call it NULL, which I find baffling having used R.
It’s basically Wild West out there. There are no rules, there is only chaos.
You don’t have to until you start wondering why a particular value in your program is NULL instead of a number. Then you realize that the program is trying to do something completely absurd like find the tenth number of a vector that has only 5 values.