Along with the aesthetic problems of mold contamination (musty odours, ugly patches), you must also consider the structural damage mold can cause. Mold's main fuction on this earth is to break down dead, organic matter. When mold spores make it into the home or workplace (they are always present) and are given the right environment in which to thrive, the mold will treat the building components (studs, insulation, drywall etc.) as it's food source causing the structure to break down.
Microscopic view of Aspergillus.
Microscopic view of Penicillium.
These are just a few of the common moulds found in homes and businesses.