Moisture content in filaments can cause a multitude of problems from inconsistent extrusion to poor surface quality and ultimately delivering a printed part with weak mechanical properties due to poor layer adhesion.
In environments with >20% humidity, water molecules are quickly absorbed into the filament forming polar bonds with the polymer chain. When the filament passes through the hot end the moisture rapidly expands, boiling and rupturing the filament. This causes bubbles in between layers, inconsistent extrusion due to material expansion and poor surface quality due to the fluctuating flow rate.
Materials such as PVA, Nylon and PolySmooth™ are particularly susceptible to moisture absorption, however other materials like ABS and PLA are also hygroscopic.