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— Puppet General Hammond, Stargate SG-1 episode "200"An element is included in a work, just because the element makes the story better in some way simply for having it, similar to adding spices and seasonings to food. These elements don't drive the plot or have a narrative purpose, and aren't genre staples. They may sometimes add to characterization or World Building, but mostly it's because, to paraphrase the above quote, having these elements is so much better than not having these elements. Now even if these elements have some importance to the story, it's similar to a MacGuffin in that the form needn't be any of the elements, but it's still better to have them.
A Super Trope to
These animals are so much better than without these animals. Or, depending on the animal, much worse (adds danger and suspense).