True, most decor is to make the enclosure look "natural" and appealing to the owner...especially since it's all they really have to look at since BPs hide all day.

Greenery, however, adds places for them to hide. Logs and such, as was said, give them shed aid spots. And if the enclosure is a bit large, the added decor closes up the open space and makes them feel more secure.

It really all depends on the type of enclosure. Tubs in racks usually don't need a lot of decor because you can't see in as well and the tub is usually snug enough. Display cages are meant for display, and so should look nice. And tanks are often larger and so need the decor to fill in the space.