Why would a P-Trap be required before an air gap for a refrigerator drain?

Question

Background:

A walk-in refrigerator with a remote refrigeration system includes an evaporator coil that requires a drain. The typical installation is an indirect drain (terminating above a funnel drain or floor sink to provide an air gap).

Location: USA and Canada

Observation:

I have recently seen installations and plans calling for an S-trap in the indirect drain line BEFORE the funnel drain/air gap.

[Evap Coil] ===/ /=== [S-Trap][Air Gap][Funnel Drain]====>

Image Source: HeatCraftrpd.com

My understanding was that traps provide a barrier of water that blocks gasses from entering the room through the drain.

Question:

What purpose does a trap serve immediately before an air gap?

Answer 1

It's preventing air movement through the drain pipe. Without it, cold air from the refrigerated space will constantly flow out the pipe, and warm air from outside will constantly leak in - wasting your refrigeration energy/expense.

So, it's doing exactly what you understand, but your understanding was perhaps limited to the case of "sewer gas" while in this case that's handled beyond the air-gapped drain, and the "gas" being prevented from moving is air - warm or cold.