Wow okay
The most common scenario in which this is useful is any game with a period during which players are not supposed to post. On any other forum architecture, the self-locking ability would normally be used to accomplish this. However, on Discourse, you may either get a mod to lock your topic (which can take time, prevent you from posting [adding that to OP], and can result in you trying to get your topic un-locked but being unable to find a member of the staff that is online, trapping you), or risk players posting and ruining the integrity of a game that may have taken weeks or even months to plan. Both are scary propositions, so for this particular purpose self-locking would be extremely welcome.
Here’s a few examples I could find quickly:
Rule 7 of this post from ChiefDelphi explicitly calls attention to this fact:
Unfortunately, it still ended up happening:
Twice:
While neither of these were game-ruining posts, I have seen some before that have absolutely been just that (been a few years and honestly can’t find them now).
If you want another example I can dig more when I get off work.