Be able to put plugins after fader. I remember, some Linux DAWs have this option (Ardour?)
Post-fader plugins position is useful for immitating analog workflow by putting clipper/saturator after fader, which means if you exceed 0db channel will start to softclip\saturate. You`ll be more careful with gainstaging (like true old-school analog guru), but most important - raising gain will also make your track sound more saturated. This is whole new dimension ITB mixing
This is only one example how you can use postfader plugins. If you have creative ideas how to use it - write in the comments please