It’s simple… Screed is used to provide a clean straight finish edge. If you choose to use screed to provide an easy method to create the clean finished edge along the bottom of the exterior stucco/plaster/etc wall, a weep screed (drain holes) must be used. If a screed with no holes is used for this application, the screed will seal the bottom edge of material and prevent moisture in the wall from leaching out the bottom edge.
So yes, you do not need screed to make the stucco wall system work, as long as you’ve created a water proof membrane behind the stucco wall. But if you use screed at the bottom of your stucco wall, it better be weep screed.