Like humans, all plants need nitrogen. The same as us, they have all kinds of molecular structures that require nitrogen. A few examples: DNA, RNA, Amino Acids, ATP.
Nitrogen is abundant in the atmosphere. About 78% of the air we breathe is composed of nitrogen. But that nitrogen is inert. It doesn't combine with other elements. So it can't be incorporated into the the molecules of life like DNA and amino acids.
Insect-eating plants like the sundew live in soils that are especially low in nitrogen. They have even less nitrogen available to them than plants that live in more favorable conditions. How have they responded to this environmental challenge?
Sundews, pitcher plants, and other insectivorous species capture and digest insects. The protein-rich insects (and sometimes larger animals) are broken down into their component molecules. The amino acids and other protein-rich molecules become available to the pitcher plant, which absorbs the nitrogen and uses it for its own purposes.
The requirement for micronutrients reflects the nutrient-poor ecosystem. Our insectivorous plants create their own tiny ecosystem, passive, noiseless, emission-free. A simple, elegant response to a complex ecological problem.