I remember being fascinated when it was first pointed out to me that "fist" is both a noun and a verb. A fist is a hand clenching. Likewise, you can sometimes put two or more objects together and they become one object, even if they are still acting independently.
I thought about this as I was playing Fallout 3 the other day and saw a small herd of cattle roaming down a street. Until that moment, I had played the game for over 40 hours and had never seen a cow grouped with another. Merely by grouping four or five together on a casual journey through a ruined city, Bethesda's developers created a new and memorable experience for me.
The point is that developers don't always have to create new visual assets or even tweak existing assets to produce new content. Sometimes all you have to do is arrange old content in a new and meaningful way.