They are to control cracking in the slab as it cures-you may not have any in your basement. If the slab is in good shape just apply your membrane over the whole surface.

Control joints are cast-in-place (look down the next time you're on a sidewalk), or sawn after the concrete has hardened (often on a 10' or 12' grid). It's expected that a concrete slab will shrink as it cures and inevitably crack. The control joints will (hopefully) limit the cracking to the grid and not occur randomly, at angles, etc... Fiber additive or welded wire mesh is embedded in the concrete to help prevent the now separated pieces from moving vertically with respect to each other. The uncoupling membrane prevents the cracks from telegraphing through bonded materials like tile. On large slabs, expansion joints are required, and they should be continued up through the tile as a caulk joint.

Do not run Ditra or the fleece products like Redgard/Custom up the wall. Kerdi can be run up the wall in some shower systems.