Why is a pre fade aux used for monitoring and a post fade aux for effects?
A PRE Fader aux is sent out BEFORE the volume fader (and usually the mute)... so any volume changes made on the fader are not reflected in the send. When monitoring, this means you can have a different mix going to the Monitors then the Mains... for example VO talent might want a guide track in the headphones to help with timing, but you don't want to hear that in the control room because it would make it difficult to asses the quality of the recording. Cue Beeps, and various music feeds would be the same principal - the talent might want to hear more or less of them than the folks in the control room. Another use in Post would be to route a track to more than one location- for example for undipped stems, or for specific processing in a Dialog chain.
POST fader sends are sent out AFTER the volume fader, so volume changes are reflected in this send. Normally Reverb sends, for example, are set up this way so that as the volume of the sound goes down, so does the Reverb. If you used a Pre Fader send, then as you turned down the track, the reverb level would remain constant, essentially changing your wet/dry mix - this is sometime useful as an effect, but not normally what you would want.
+1 to Sonsey's explanation of Pre and Post-fade sends.
However, I'd add that you don't ALWAYS send monitor mixes prefade, either.
I have a large vocal ensemble that I mix regularly, whose monitors sends I almost always send POST-fader. Why? Because I want them to hear in their monitors the same mix that the house is hearing, so that they can more accurately adjust and balance between each other. I also want their monitors to fade up and out with the house mix (for a variety of reasons).
The key take-away here should be that there are no hard and fast rules. Learn what your tools do, and then choose the best one available to accomplish the job at hand.
If you are clear on what PRE and POST-fader sends do, how they differ, and the relative tradeoffs you have to deal with when using them, you can make an intelligent decision to use either one based on your needs at that specific time.
In a building changing levels on the main mix only will change the sound in the main mix if you don't change the levels in the monitor mix together in most rooms, EQ all speakers the same and go (Post Fader) All Aux sends or Monitor sends set at unity, And all adjustments will be made at the main fader, Made my sound 100% better, The trick is perfect balance between monitor mixes and House Mix.
Keep some head room in the pre' so you can raise volume wherever in the mix to make everyone happy.