The one scenario I can see where it would be an advantage would be if a crook was grabbing one's weapon and one had a backup piece. In that case, drop the mag, let go of the gun, and shoot the crook with the backup gun before the crook can restore his newly-stolen gun to function.
“The one scenario I can see ...”
That’s the scenario I’ve seen most often.
Another may be peculiar to here in Ohio where you can’t carry everywhere. Some places you can avoid, but some you can’t, at least not easily.
If you want to go to the library, where you can’t carry legally, or go to an eatery that serves alcohol (where you can’t carry legally) so that some of your companions (not you of course) can drink, then you can drop the magazine, store it in the vehicle, and on the one hand you have not left an immediately operable firearm for someone to steal while on the other hand you can make it operable very quickly.