'Post (after) means the wet/dry mix ratio you set at the aux send stays the same as you move the fader.
Then you can see what would happen in the 'pre position as you move the fader up or down. That could be useful in some cases.
Generally speaking I run stuff like reverb, delay etc... in post fade. Occasionally though the track and workflow calls for prefade. It really depends on different scenarios and how you do things