But what does this even mean? Why would anybody do this?

I would guess that it is maybe just a simplification of a different case; using for as a topicalizer can be pretty useful. Clicking through the commits seems to show that the change to scalar was a bugfix resulting from the discussion in #24346. I found that an enlightening point was: "Since scalar is just a directive to change context, I don't see why it should change *anything* else."

In general, I think what this boils down to is a problem caused by autovivification, which, while of course powerful and nice, is also sometimes the source for such confusion. There is of course no autovivification; from CPAN, but personally I like to just be explicit and use exists and friends to avoid things vivifying when I don't want them to.