Extruders come in quite a range of configurations (single screw, twin screw, octascrews of all types...), an even larger range of geometries (L/D's all over the map, intermeshing twin screws, non-intermeshing twin screws...), all of which have a purpose.
The puzzle to me that I've noticed over my career and client visits is that companies tend to fixate on using just one type. Some companies only use single screws ("twin screws are too expensive and we can modify the screw to do what we need") while others use only twin screws ("can't do a blasted thing with a single screw"). And then at that, they only seem to run in a single dirction, either counter- or co-rotating, but not both. What is really funny is that I've seen "twin-screw companies", when they do have a single screw, run it like it's a twin-screw: starve-fed, instead of flooded as is typical for single screws.
I can understand a company locking into a business model and following it religously, but extruders?