I've no evidence this is being used by The Economist's app - but this is "usually" the reason for this feature:

- https://www.wired.com/2016/11/block-ultrasonic-signals-didnt...

(not trying to spread FUD, endorse this practice or criticise in anyway, just trying to help the parent comment understand one potential "excuse" for an app "needing" this functionality for a non-obvious use-case.)