[Tagging] traffic_signals:direction=* vs. direction=*

[Jean-Marc Liotier]

traffic_signals:direction=* is used on 27278 highway=traffic_signals
objects:
https://taginfo.openstreetmap.org/tags/highway=traffic_signals#combinations

highway=stop is combined with a direction tag on about 77000 objects
(direction=backward, direction=forward and the literal direction=*)
https://taginfo.openstreetmap.org/tags/highway=stop#combinations

highway=give_way is combined with a direction tag on about 43000 objects
(direction=backward, direction=forward and the literal direction=*)
https://taginfo.openstreetmap.org/tags/highway=give_way#combinations

Given how widely used the direction tag is for highway=* signs, why
isn't it also applied to highway=traffic_signals ? Does
traffic_signals:direction=* bear additional meaning that direction=*
does not convey ?