The corner and middle stakes hold the two boxes togther and help secure the raised bed in the ground. The top trim keeps the sides straight and the top cross pieces were used to keep the sides from pushing out after the soil was placed in the raised bed. I will be building another version to refine the design a little bit. The intent is to use the raised bed for a square foot garden thus the top cross members perform two functions. 

Since it was close to Mother's day when I made the raised bed, it became a Mother's day gift. The next version will have the long top divider sections. I have also drawn up plans and will make those available when they are finished.