It's just there to verify flow. Call it cheap insurance if something went wrong during oil change. No flow out that screw = seized engine after a few miles of riding. Loosening a screw is better then buying a new top end.
How much grab did that magnet have? Unless it is a magnet made for high temps it will keep getting weaker each time the bike is run.
My magnet had a tiny bit of fine metal goo on it after 1k