TopBunk that's a pretty crap attitude to a reasonable question (apart from not specifying the type, although the name seems to have given it away).
"All you need to know is the SOP for dealing with a particular problem ... in this case an engine fire. It is for the regulatory authority to worry about the effectiveness of the necessary equipment. If it is approved, then good, follow the drills."
Where did anyone ask about effectiveness and necessity?
Checking the "bottle" for each engine is part of our walk around, it has a little gauge on the front showing the pressure, so we have to know where it is.
If I flew a type where it wasn't part of a walk around I would still be interested in knowing where it was, how many there were, what agent was in them, the amount etc