The answer is, indeed, the adverb "very".
In the sentence "Wetlands are very useful to remove pollution, because the soils and plants can filter away poisons", the adverb of degree "very" is modifying the adjective "useful".
Adverbs of degree are words that may modify a verb, another adverb, or an adjective, changing their intensity. Saying something is very useful is not the same as saying it is useful, because "very" shows a higher degree of intensity or, in this case, of usefulness. We would rather buy a product that is very useful than one that is simply useful.
Other adverbs of degree are, for instance: too, quite, extremely, enough.
