We have two kidneys in order to balance the load placed upon them by the body. The body can manage with only one kidney but the job is faster and easier with two doing the work. The sooner toxins and waste are removed from the body the better!
There is also the issue of body balance and symmetry. If we had only one kidney then one side of the body would be slightly heavier than the other, requiring more balance adjustment to walk and run evenly. It is also good to have two kidneys in case one should become damaged or destroyed
It is also possible that there is an evolutionary advantage in having a spare kidney, and this is certainly important in modern medicine, because people can live live normal lives with one normal kidney.
Sometimes having a single kidney does affect your health, but the changes are likely to be small. Over long periods of time, however, these gradual changes may require specific measures or treatments. Changes that may result from a single kidney include High blood pressure, proteinurea, reduced glomerular filtration rate etc.