Paying for miniaturization.
If you set up a basic common "Comms" program routine
in a micro, to Receive (external) Output commands and Send
INputs back to the "Main" Controller, effectively you have created
Remote-I/O.
If you have an SD card CPU, even easier.
Your "Micro Remote I/O" could become a standard device
in your facility, with a few spares on the shelf.
Occasionally you might have the appropriate I/O points
on the Micro, to fit the need.
Your aren't having to do anything harder than trying to
set the IP, Subnet, Rack size, etc that already has to be
done with traditional Remote-I/O.
OEM's can get significant discounts on pricing, given the
volume.