Why do you think it is necessary too have 2 bones in the lower arm instead of just one?The lower arm has 2 long bones, the ulna and the radius.

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.