I bought a house, had a sink in the garage, city told me I need to have permit and add a vent for the sink. Why do I need a vent for utility sink?

I bought a house, had a sink in the garage, city told me i need to have permit and add a vent for the sink. Why do i need a vent for utility sink? Because every fixture that has a trap MUST have a vent,
Let me explain about vents. When you pull a plug or flush a toilet the water going down the pipe sets up a suction. This has to be relieved by outside air. Hence a pipe through your roof. If the vent is blocked by a birds nest or a dead critter the water will pull suction through the nearest trap and you will hear a gurgle. It may be a fixture trap or the toilet may talk to you but you will hear it someplace. While it is possible for the suction to pull enough water out of a trap to have sewer gas escape but it's more likely a smell will come from decaying food, grease or hair. A blocked vent is unlikly to cause oders. And now you know about vents. Good luck, Tom