can be a huge difference...

for instance... take a simple volume pedal....

placing it pre, will change how much input you are feeding the amp...

reducing the gain when you lower the volume... (changing your tone)

putting it post would lower the volume of your tone, keeping the gain intact and for the most part your tone too.

there are different reasons you might want one over the other... no right/wrong

another good example is the looper...

doing it pre... loops the guitar without amps/effects...

then you can put the guitar down and dial in your tone while the loop plays...

putting it post records amps/effects allowing you to change tones and solo over top of it...