My guess would have been that weak, very non-humanoid devils would want to become fiends in order to infiltrate the human society better. Whether that works or not is another question, but I doubt weak devils would be able to think that far anyway. Possibly a weak devil would become stronger in the process as well. It's hard to see a strong devil becoming stronger. But then again, from all the xianxia I've read, I suspect also in East Asian traditions the human form could be considered the ultimate one, so perhaps it simply is desirable for the devils, no questions asked.