To learn! Everyone should write a ray tracer (or three) on their own before even attempting to dive into a big open source production renderer. An established high profile project isn’t a great place to learn the fundamentals, since those parts are already done, and they don’t necessarily want people contributing who aren’t ready for it or are just there for personal learning purposes. It’s not fair to call it ‘reimplementing’, this project isn’t trying to be Blender. There are loads of other reasons to write a non-Blender renderer aside from learning, including writing one for games or interactive or scientific applications, needing special features, doing research and experimenting with rendering algorithms and data representations, needing a very small/fast/light renderer codebase, etc. etc.