The answer is the ones I cannot fit anywhere else. There are so many features you could add to a channel but the available panel space is limited. Right now I really have two mixer options. The lower cost one does not have a separate routing module but includes basic routing in the channel module (a couple of AUxes, PAN and a combined Mute/On/PFL or Mute/On/Solo switch. There is only a single stereo bus.
The more complex one does have a separate routing module which allows all of the above plus 2 more AUXes, an insert switch , routing to one of two stereo buses and allows the channel module to include a HPF.
If the simpler wants wants to include any of these additions the only place for them is in the fader panel. I do not plan to put AUX pots there but a bus select switch and an insert switch could go there.
Cheers
Ian