Mold causes concern for three main reasons:

Unlike, say, pollen or VOCs, mold is a living organism that grows, so a small problem can become a large one fairly quickly.
Mold causes allergy and asthma-related symptoms, but it has also been linked to serious health conditions such as aspergillosis.
Some species of mold produce toxins. Although these are not typically produced in large enough quantities in buildings to hurt people, some of them are quite potent.

Strictly speaking, the presence of mold is not a problem in itself. Mold spores are everywhere, even in a reasonably healthy indoor environment; you can’t get rid of them completely no matter what kind of remediation you perform. But if you have moisture problems that can contribute to mold growth over time, or if you see mold growing on drywall or wood surfaces, you likely have a problem that requires attention. If mold spores aren’t already airborne in significant quantities, they probably will be soon.

To determine what a “significant” quantity is, Eagle compares samples of indoor air with a sample of air outside your home. Outside air in most circumstances contains typical levels of airborne mold for a given location, and therefore shows you what levels you should aim for when conducting remediation.

Of course, if you’ve become overly sensitive to the presence of mold, your nose won’t care what levels are like outside—it only cares that it detects a problem. Unfortunately, remediation can only reduce mold to acceptable levels compared to outside air; it can’t eradicate mold completely or prevent mold outside from getting in.