Why must AC filter be installed in a particular orientation?

[goshenHeat]

Please satisfy my curiosity by answering this question about the filter I use on my whole house AC. This filter is blue with a cardboard perimeter. On the cardboard there is an arrow pointing in the air flow direction. I cannot see a difference between the two sides of the filter. Why must the filter be installed in a particular orientation?

[HayZee518]

aair filter

the filter medium is coated with a dust catching substance.

[LazyPup]

Also. although you can't see them, there are reinforcing threads on the discharge side to reinforce the medium so it won't blow out.