AN fittings and hoses are often found installed on high performance and race cars. AN hoses are used for fuel, oil, coolant and hydraulic fluids. AN stands for Army-Navy as the specification was developed based on a standard for flexible fluid connections used during World War II. They are often used in aviation but like most high performance aviation equipment they eventually ended up on race cars and then street cars. 

People use AN fittings because they look cool and provide a secure fitting that seals hoses better than traditional push on nipples and hose clamps. They form a serviceable connection that provides a reliable seal. AN fittings can also be used for leak free bulkhead pass throughs like those found on the outside of fuel cells.

Youtuber Carter Maneki's video about SPELAB AN hose