What would be the benefit of having a third healthy kidney?

I read somewhere that when they do a kidney transplant for people with failing kidneys they usually leave both of the originals in and just add in the third “in line” with the other there to filter the blood supply. The logic being even if the other kidneys only have say 5% functionality then hey, that’s 55% functionality overall, assuming a perfectly healthy transplant kidney. However… if your current kidneys already worked just fine and you added a third, you’d have 150% of the normal filtering capacity of two healthy kidneys.

So what would that do for you? Would you just be really good about filtering your wastes out of your blood? What would you be able to do differently that most normal people couldn’t? Would there be any drawbacks?