It's possible to do a scene with only one purpose though, as long as it is not set-up for future events. Set-up must never ever ever go on its own because it's not interesting in itself. . . .
And, come to think of it, foreshadowing comes very close to it. True, for foreshadowing you want to make the reader worry on the spot whether something bad will happen, so it's possible, but still, something has to happen in the present day.
But I send the heroine through her native land, and I realize the witch is still about, and this would be the ideal time to warn the readers that unlike the other plagues and nuisances she has had to deal with, this one is coming back. So let's have an encounter! Nothing serious enough to undermine the future events. . .
And the outline, naturally, stops dead in the water because you can't just have "something happen" -- something must actually happen. Preferably foreboding. . . until finally, after much wrestling about what the witch could and could not do, I realize she can do nothing, just send the heroine packing because she wants nothing to do with her, and creep her out in the process. Let's see how it works.