Locking is mainly used for extreme cases where the community abuses powers (with or without realizing it) and a moderator has to take action to prevent it from creating a bigger mess. Often it is just people being uninformed. These are situations when a post should be locked (from the FAQ):
When should a post be locked?
Posts should generally only be locked in cases where something
seriously bad is happening. In particular, where the ongoing updates
and edits are actively detrimental to the system.
Some examples of when a post might be locked include:
- A question or answer where repeated voting or editing is happening in a way which attempts to game, hack, or otherwise abuse the system.
- A question that gets opened and closed repeatedly many times without achieving community consensus on whether it should stay open or
- A question that, for whatever reason, continues to attract flame posts, spam, or other inappropriate answers.
- A question that is repeatedly vandalized by its asker; for example, to drastically alter the meaning of the question that invalidates
existing answers, or to obliterate/obscure the question.
- Another type of locked post is a merged question. When a moderator merges two questions that are exact duplicates, all the answers are
moved to one question and the other question is left as a "stub," with
no answers. The stub question is locked.
All of these, except maybe the last one, are cases when voting, comments, and edits are still harmful. You could argue that there should be an option to just lock closing/reopening of a question, but that's too rare for the dev team to waste resources. Not to mention, a post so controversial might cause an avalanche of downvotes and mean comments.
If you can provide a list of questions that benefit from this feature, I might change my opinion, but there doesn't seem to be even a small amount of questions that would truly benefit from this.