As good as the Y-20B is, it still could not carry some of the larger vehicles like TELs for ICBMs, HQ-9B/29 etc advanced air defense launchers and even some larger radar vehicles. Also niche cases like satellite delivery will require oversized transport aircraft. If PLA wants to be a strategic force and deploy anywhere if required, an transport of this size and range will prove to be useful. Also as far as I know, although there aren't any leaks/news about the transport specifically but the 250kN class turbofan is a confirmed project by Shenyang AECC, such engine is in an odd spot since it is too small for wide body airliners but too big for narrow bodies like C919. Such class of engines were used on aircraft like B757 but these are mostly obselete and replaced by more advanced wide bodies like B787. IMO the only likely use case for this engine would be a C-5/An-124 class transport both used engines in the 225-250kN class. This also has been the consensus of credible PLA watchers like SOYO(who posted the original tender for the engine) and Guancha trio.