Stickers or stickering you lumber is done with clean, dry strips of wood. Normally species that may stain, such as red oak, are not used. The stickers are used to allow air to freely circulate around each board. Not only are they used in lumber processing at the sawmill during operations like kiln drying but woodworkers take the time to sticker whatever pieces they are working on whenever the work pieces will sit over night or longer.
If boards or work pieces are stacked directly on top of each other or in contact with a workbench or other solid surface, one face of the piece on top will be exposed to the open air and the other not. This can often cause a board to warp. In a stack, pieces can exchange moisture with each other. When you pull out a piece and it's exposed on all sides, it may warp.
Wood is the stalk of a plant and it's job and physical composition is to hold and transfer moisture. Even dead and sliced up, the cells are still good at doing that job. It's considered a prudent precaution to allow air to circulate around all surfaces to try to keep an equilibrium. Whether you have a pile of boards in storage or a stack of work pieces, keeping stickers between the individual pieces, spacing them to prevent sagging and keeping them in a vertical line is standard practice.
I have a picture of a pile of drawer parts in various stages of construction. In the foreground you can see the pile of sides with stickers. I keep the parts stickered through each process until they get assembled. I would be a pain to have even one part warp and not be usable, particularly since several processes have been performed on each and all those would have to be set up and done again. Sticking isn't a guarantee but it's proven better than not doing it.