Leave some way to clear blockages, either with high pressure water from a garden hose, or coat hanger wires. These things which carry only water find some way to plug themselves up.
There may also be an overflow designed to alert the homeowner that the normal pipe is plugged; it usually drains outside somewhere.
"Probably the most misunderstood portion of a condensate drain system is trap installation. The primary purpose of a condensate trap is to prevent air from moving in or out of the coil box or air handler during operation. Traps must be installed in a manner that will stop the air from passing through, but still allow the condensate to drain from the condensate pan."