Both of these options eliminate the chances of melting silicon joiners that are close to turbo housings & manifold runners. The main reason a cast elbow has a advantage over a standard radius elbow is the CLR. (what is a CLR) This is a lot tighter on the cast elbow meaning for tight spaces they are an ideal option.
The cast elbow also has the same surface finish as a turbo compressor cover so cosmetically the cast elbow matches the comp. cover