On the preamp page you can assign an input socket to the selected channel by selecting the source (local, SLink, USB…) first and then adjust the socket number. Currently the socket number is always reset to 1 when switching the source. So when the source is set to local 21 and you want to change it to USB 21, you first have to change the source type to USB and then you again have to adjust the socket number from 1 to 21.
But it is not unusual that you want only change the source type but the socket number should remain the same, e.g. in a Studio situation, similar to the traditional switch from mic to tape input.
I know it might be done also like it is and there is also the IO screen, but you have to do a lot of steps to do it instead of an easy switch of the source type.