The Fog pass device doesn't just identify signals. It gives indications for a whole range of landmarks like warning boards, TSR/PSR locations, engineering restriction areas etc.

Loco pilots, no matter how experienced, are not superhumans, to remember the position of every signal on the route (and there will be hundreds of signals, many with different aspects). Especially signals in the middle of nowhere, far from stations and with no landmarks near them, are more difficult to remember when an LP is running a train for hours on end.
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Engineering restrictions on every route also change daily. Whenever an LP is starting a journey, they are given documents indicating the locations of speed restrictions, engineering restrictions and potential issues (non-functional signals, repairs etc) that are in place at that time. These things can be at various locations along the route, and it is unrealistic for an LP to easily identify all the locations. The Fog pass devices make it easier to realize when the LP is approaching these restrictions by giving distance callouts all the time.

They are indeed useful devices for LPs. They are being procured in large numbers because the staff find them useful. Please don't dismiss such things without looking into the details.