There's a fixed, and rather small set of lock reasons, with fundamentally two different types - post and comment locks. Maybe it's meta, but a lot of the time our decisions when it comes to locks may need a bit more nuance - and it's pretty essential folks understand what is happening and what we would like folks to do or not to do.
Sometimes we have exceptional situations - that don't quite fit into the boxes - we may want a historical lock for a feedback post that's no longer needed for new feedback. It's not off-topic, it's just unwieldy. We might want to use a lock and have a post notice for some particularly controversial post (hopefully not that often).
Considering locks should be exceptional, having a way to customise the post notice lets mods enhance communication (least one way). I'm currently abusing comments to do this, and it's a bit lame.
