Alcohol is basically a toxin that your kidneys have to filter. Moderate drinking doesn’t make this job too terribly difficult, but excessive drinking puts a real strain on them. Too much alcohol actually changes the function of your kidneys, much like it changes the function of your brain, and makes them unable to effectively filter your blood.
Alcohol is also quite dehydrating, and water is critical to the function of kidneys as well as every single cell in our bodies. This internal struggle is one reason that hangovers hurt so much. It is really important to drink plenty of water whenever you’ve consumed alcohol.