"what is the point of doing this with these upper 8 bits?" The other way around. There is no point doing anything special with how upper address bits are routed within the CPU during IO operations. There is indeed not much different between say LD A,(BC) and IN A,(C) other than the IORQ control signal.

There are 12 16-bit main registers connected via a gate to two registers PC and I/R connected to a 16 bit inc/dec circuit connected to 16 bit address latches. Thats it. There is no special logic handling the upper 8 bits different.

However, there is a difference between IN r,(C) and IN A,(#n). The first one select the BC register pair, the latter select the hidden WZ pair which got previously loaded with A and the immediate operand #n.

Here you can study layout and behavior of the Z80 CPU at transistor level https://floooh.github.io/visualz80remix/https://floooh.github.io/visualz80remix/