> But why use multiple exposures in the first place? I don't want to be Man Ray with his photo collages.
There's more than one way to use multiple exposures.
Yes, it is possible to do this in post. One reason I do some of these this way, frankly, is that I'm tired of the criticism I sometimes get for "non-photography." So for things like this, I sometimes do it in-camera. Honestly, it's really a terrible reason. I should just do it the way that's best, and forget what other people say. In these three cases, it's probably just as good or possibly even better to do it in the field with the camera. I'd say it's better to do it in-camera since one can easily judge the result in the field and reshoot. Each of these is one of a number of attempts - usually four or five at a time - and they're not exactly precision craft. This is particularly true of stuff like #2 which might be emphasizing things like flowers blowing in the wind.
> I thought it was a remedy for high noise when shooting in dark places.
You could do that, although it's better to do it with some other software, which assumes that the images are supposed to be equal, thus eliminating noise - which would be different in every exposure. Sort of like the Fuji x10/x20 do.