Not entirely sure but the wiki says it "is applied where cycling is legal, not obvious from other tags and where it is not explicitly designated for cyclists".
I have added it to some useful footways/paths in OSM (so it is rendered and routers can use them) like this: https://www.openstreetmap.org/way/51795132 - there's no shared use sign but a) it's a core path and b) access rights would also apply as it's not adopted.
I would agree that if it's not able to be cycled then there's not much point in adding it, as technically you could add bicycle=yes to all paths that aren't adopted or pavements.