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I believe that the reason this is necessary is so that our lower arms can be more easily maneuvered. Think about how much more easily you can move (and especially rotate) your forearm as compared to the way your shin moves. The forearm has two bones, allowing it to rotate, while the shin only has one bone and therefore cannot rotate nearly as much.
If we only had one bone in our forearms, our forearms would presumably be as limited as our lower legs. This would make us much less dextrous and much less able to use our arms and hands in as many ways as we can.
It makes our forearms more versatile and flexible. Having two bones is simply superior engineering, as we do so much lifting and moving with our arms, the upper arm provides the muscle strength while the lower arm provides dexterity of movement.
Take the weight we put on our arms and it is distributed over both bones, which allows us to move more efficiently from side to side, and forward and back to achieve the tasks and work we do throughout our daily lives. It also takes pressure and stress off of our wrists and elbows by distributing it more evenly throughout the forearms.
We have two bones so that our wrist can pivot at the distal end. This is a similar characteristic of all tetrapod vertebrates. Current evolutionary thinking suggest that the Rhipidistian Crossopterygian fish is our common ancestor once they were able to breathe on land. The same thing can be said of the leg. The two bones in the lower leg allow the ankle to pivot, thus providing us the ability to move fairly well. These characteristics are all related back to our ancestors, in the same way that we have eyes in the front of our heads and not to the side. Our neck can pivot.
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