It would seem there are a variety of reasons why mods might act on a thread, and these have to be considered to determine the best course.
In some cases, a temporary lock while the thread remains in place seems appropriate. These might include threads that contain tangential posts that need to be split off or deleted, or threads that have begun to get snippy.
In some cases a hide is more appropriate, even necessary. If it is believed that there is mis-information or a copyright violation, the thread should be removed from view until the correct action can be taken. It would also be a good action if the thread went beyond snippy. The idea of a thread re-direct is a good one if that feature is available, as is the idea of creating a locked stub thread that will be quickly found in the same forum as the original. At least, that's the first place I looked.
I see occasional post editing when names that shouldn't be released are posted, and that seems a reasonable course. No reason to lock a thread for a quick update.
Why else is moderator action taken?