Air Gap faucets are used almost exclusively for Reverse Osmosis (RO) Systems. Although not all RO systems use them.
This faucet has 2 hose barb connections on the bottom, in addition to the purified water connection, to connect to RO drain lines (see photo below). The drain water comes from the RO system up to faucet and then it passes through an air gap. The water then goes into a different line which is connected to the drain. The air gap provides protection from drain water backing up into the RO System.