Exactly, junction relations in general are useful for geocoding and analysis purposes (and sharing links to other mappers), but there isn’t an obvious benefit to routers.
Some routers might penalize give way (yield) signs very slightly, less than stop signs. But just as with traffic lights, any penalty would be watered down by the fact that you don’t end up having to slow down 100% of the time. So I’d expect it to be a negligible penalty anyways.
Assuming you’re talking about visibly posted signs or markings, the more interesting thing about highway=give_way is that a navigation application’s map could mark it as a safety reminder. Mapbox-powered applications do this, for example. At the zoom levels we’re talking about, I’m not sure the precise location particularly matters, but highway=give_way has to be tagged at the position where the vehicle pauses, since it doesn’t apply to all the incoming ways equally.
You’re probably right. Any router that’s so detail-oriented as to care about right of way would probably be consuming highway=give_way anyways.
That said, most routers do use junction=roundabout on the way as a signal to switch to roundabout-style guidance instructions. Even if the guidance instructions include turn lane information, users would still benefit from hearing something like, “Take the left turn lane through the roundabout,” instead of just “Take the left lane to turn left.” And even interpreting these turn:lanes values holistically based on the whole roundabout geometry, instead of dropping them as invalid, requires some indication of a roundabout.
Besides, I think it is good to aim for completeness with respect to intersection control devices. For example, any future StreetComplete quest about the form of traffic control at an intersection would rely on the fact that there’s always something to be tagged there. However, this is more of a consideration in regions like the U.S. that much more frequently post intersection control signs, particularly stop and yield signs.