The Bladderwort Test

Transposable elements are found in every living organism, usually represent a large fraction of the genome and
are correlated with genome size.
They are classified into many groups and contribute to genetic and epigenetic variability in the genome (also this and this and this), which is important for evolution, as it drives speciation.
They are thought not to contribute to fitness (or to contribute negatively), but that is not to say that they don't take part in the evolution of a species. Thus, I think that whether or not they are functional (and some are, constituting the promoter regions or fine-tuning certain genes through RNAi) they still contribute to the diversification of life, which makes them "important" (I guess).

From Sarah P. Otto (2007):
"Although cell size typically is larger in polyploids, adult size may or may not be altered; as a rough generalization, polyploidization is more likely to increase adult body size in plants and invertebrates than in vertebrates (Gregory and Mable, 2005; Otto and Whitton, 2000).
The poor correlation between cell size and organismal size was even remarked upon by Albert Einstein, who wrote Most peculiar for me is the fact that in spite of the enlarged single cell the size of the animal is not correspondingly increased (Fankhauser, 1972). The key to accurately predicting the effects of ploidy on body size must come from developmental biology. In cases where
morphogen gradients guide development, ploidy need not affect adult body size (Day and Lawrence, 2000) because ploidy need not alter the overall density of cellular material, only how it is packaged (i.e., into cells that are twice as large and carry twice as much DNA). By contrast, where growth is determined by cell-cell interactions or where there is a fixed number of cells in the adult, ploidy, by altering cell size, should directly influence adult size (Gregory et al., 2000)."