in biology, no one is able to answer why, but able to answer how.
why do we have two kidneys? no one can answer
how does the kidneys work? that can be answered.
anyway, the above may not answer your question.
well, put it this way. why do we have two kidneys is different from what is the biological significance of having two kidneys
I do not understand the difference: We know 'How' a pedal cycle makes forward motion and on the same principal we know 'Why'. In both cases it is by someone pedaling. That is why it goes forward and also how it does.