Here's an example of one thread that shows up in multiple forums:
Harvesting seeds from your groceries
It was started in the forum
plants. But it also is the kind of thing a beginner gardener might ask, so the staff added it to
gardening for beginners. It might also be useful for plant breeders to find seeds for their projects, so it was added to that forum too.
Having many almost but not quite identicle threads scattered about the place makes it even more difficult for people to use a site like this. Let's face it, it's a pretty active forum. This way, we have one thread and it shows up in many places, making the information far easier to find.