As aircraft do not encounter traffic jams like land vehicles do, it makes sense that they do not require a horn. However, many people do not know this, but aircraft do have horns, just as any vehicle does on the ground. Pilots mainly use the horn to get the attention of the ground engineer, so they can talk on the intercom (by wearing a headset).
While horns aren't used for alerting other aircraft during flight, they are a valuable communication tool while a plane sits in a hangar or undergoes maintenance. In most cases, an airplane's horn is used by the ground engineers, performing maintenance/repair work in the cockpit and need to communicate with the ground crew.
The sound of an airplane horn is more similar to that of an alarm than to that of a car horn. An airliner's horn was once recorded and posted as a video by KLM Airlines. According to the airline, when you press the button, "it sounds like three steamboats are passing under the plane."
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According to KLM Airlines, whereas car horns are used mainly as warning devices, aircraft horns are primarily practical tools used during aircraft maintenance. It can emit signals to warn engineers when a system malfunctions or if there is a fire.
When in flight, KLM says that the aircraft horn is not working because the signalling system is off.
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