Evolution doesn't imply a cause to every attribute. Totally plausible that having one kidney is advantageous, our bodies have general bilateral symmetry of orga

Evolution doesn't imply a cause to every attribute. Totally plausible that having one kidney is advantageous, our bodies have general bilateral symmetry of organs, there is no strong selection pressure against having that symmetry result in two kidneys, hence we have two. Most of your DNA is "non-functional". Evolution does not have a plan.