They help raise the system above the bumps of the floor/the tailgate flap/etc. Additionally, my drawers don't have a bottom extrusion piece connecting the sides together - that's what I use the bottom piece of 3/8th aluminum bar for. That way I maximize storage space in the drawer.This is what I have planned for mine as well. I like the turnbuckle idea, but not certain about the wooden or aluminum cross members.
Have y'all found those to be significantly important to security and stability?