I recently learned that you can't ready bonus actions, because you can't ready any action that can normally be performed only on your turn:
A bonus action can be taken only on your turn. You, therefore, can't use Ready with a bonus action.
This official change to the rules confuses me. It seems to say that specifically because you can't use a bonus action out-of-turn, you can't therefore ready it. But you can't use actions or moves out-of-turn either, so the same logic seems to invalidate most use of the ready action, leaving only reactions readyable. (It doesn't reason that you can't ready a bonus action therefore you can't use it out of turn, which would appear consistent with the rules.)
Furthermore, readying reactions is only useful if you want to delay a reaction that normally happens before its trigger so that it instead happens after its trigger, which is both rarely useful and not the sort of thing that fits with most of the official rules-changes/rewrites the design team has poured out thus far.
Are actions or movement normally usable off-turn? Is there any use for Ready beyond reaction timing given this official rules change? Is there a more recent errata overturning this one? What's going on here?