It depends on of the user how they will use it. When I do, I do it to have an update about the topic, maybe to have a summary of the topic, maybe to have a list of the collective responses.
I would understand it if you posted a reserve as an update, but this isn't an update. It is right after you submit your post. What's the purpose of doing it immediately?
As I stated in my first response it is a placeholder. The reason for posting it immediately after the OP, is to reserve the spot for future use sometimes it's only the second post position, some people will reserve a couple spaces.
It unfortunately seems common practice on the forum for many users to not read the OP, or then digest the rest of the thread before they post.
Now if people actually read the OP, then scroll down to read the thread before posting they will come across these "RESERVED" posts - but let's say it's been a month since the OP was written, and maybe there have been changes made. If the poster feels it is necessary, useful or helpful they might log these changes in the post that was formerly just "RESERVED"
You have probably already read some topics that have used this strategy, because they don't always leave "RESERVED" in the post after editing in whatever they deemed relevant.
Others have also pointed out another possible reason for doing it, which is to purposefully but out of necessity(This is subjective) multipost an OP that benefits from being broken up.