In addition to condensation created by the HVAC system, the drain line can fill with dust particles as well as bacteria causing mold, mildew, and algae to grow in the condensation drain year-round, even with a filter.
The drain eventually can become clogged with a sludge and grime that is called Zoola, a mold riddled gunk. As Zoola accumulates in the condensate drain it becomes constrained and eventually will block the drain line. This can cause the HVAC system to back with water, flooding the overflow pan, causing water damage inside your Unit -- and to the Units below you -- and potentially leading to thousands of dollars in repair to drywall, ceilings, and/or floors.
Just as with a house, you are responsible for proper maintenance of the HVAC drain lines.