A Bitwig tutorial host walks through the practical difference between pre-fader and post-fader aux sends, using a convolution reverb set up as a distance effect to make the distinction concrete.
In post-fader mode, the send level is tied to the track fader, so pulling down the volume also reduces how much signal reaches the reverb. If you want the distance effect to stay consistent regardless of where the track sits in the mix, that coupling becomes a problem.
Switching to pre-fader breaks that link. The reverb receives a fixed amount of signal no matter what the fader does, giving you independent control over the effect and the dry level. The tutorial shows two ways to do this in Bitwig: changing the default behavior of the entire aux track, or overriding just a single channel send by right-clicking it directly.