An air gap under the sink is not always necessary, however there are certain circumstances where it may be beneficial.
Reasons why an air gap should be installed:
* To keep wastewater from being siphoned back into the drinking water supply.
* To prevent water contamination caused by cross-connections between two pipes that contain different types of liquids.
* To protect against pressure fluctuations in plumbing systems which could cause sewage to flow back up through a drain line.
In conclusion, whether or not you need an air gap underneath your sink depends on your particular situation and local regulations.