While we can live with only one kidney, it is an essential organ; if your kidneys fail, you will die very quickly without medical intervention. It is a good thing that nature gave us two of them. Having two kidneys allows them to function as backups to one another. It also means that you have excess capacity; if you need to clear a lot of fluid from the body quickly, each kidney can pick up part of the extra load, whereas people with only one kidney can run into trouble under such a physical stress. Also, if an individual develops a kidney stone, the other kidney can continue to function while the stone is passed. If there was only one kidney, stones, which are not uncommon, could kill you.
Another reason for the existence of two kidneys is the body's tendency toward symmetry. When the internal organs are developing, many of them follow a mirror-reflection pattern, which is how we get two eyes, two arms, two legs, and so on. The controlling mechanism that establishes the kidneys is part of the same system that creates symmetry on other areas of the body.
Despite being able to live with one kidney, our bodies function much better with two. Having two kidneys is most likely due to having a reserve in case something goes wrong in our bodies, such as injuries or too many toxins. As the kidneys job to help filter, it is very important that if a problem occurs there is a back up. It also is important for if we have a buildup of certain minerals that our kidneys can filter is easier by having two or to be able to have one kidney take over the work if one of them has a problem. Kidneys are in a place in our bodies that is much less protected than, for example our heart. Our heart has the rib cage around it to protect it. The kidneys are in a vulnerable position and therefore more likely to be injured. Thus having two kidneys is a great thing to have, despite people being able to live just fine with one.