I recently replaced some insulation in my home and i found out it's a lot more complicated than just put slap it in the wall. That is to say my understanding is pretty rudimentary so my explanation may not be the best.

Insulation in a house creates a temperature difference between the inside and the outside of your house. If the insulation doesn't have a proper moisture barrier this can lead to water getting trapped on the inside of the insulation due to condensation. The water then comes leaking out if your walls, making everything extremely damp. The darkness inside the wall along with the dampness makes an ideal environment for the mold, which quickly spreads outside of the wall and into the rest of your damp house.