I'm the one who originally proposed to have that close reason. When they re-did the close votes process, the SU site mods asked the userbase what close vote reasons we should pick for SuperUser that are specific to our site. See [here](http://meta.superuser.com/questions/6883/what-should-be-featured-in-our-list-of-off-topic-close-reasons/6884#6884).

It got upvoted, so they included it in the list.

Presumably, the SU site mods have the ability to directly edit the "malleable", site-specific close vote reasons, or to get a developer to edit them. They are basically "canned text"; the alternative would be for each person who wants to close a question for one of these site-specific reasons to have to type out an explanation after clicking "Other" as the close vote reason under Off-Topic.

Basically it's a convenience for having to type out a long explanation in the reason for the off-topicness. It's a fairly specific case and probably doesn't deserve mention in the help center, because people are bound to misinterpret it, much as you did when you wrote your question ("that questions about pre-release software are off-topic" -- this is not the case).