There are many reasons, mine for example beeps when I lock/unlock the car with the remote key fob, it beeps if I switch of the engine and the lights are still on, it beeps when I reverse toward something solid, they are all warnings, if you need a better answer be more specific with your question.
Car honks sound like beep beep because the purpose of car honks is to gain attention. The beep beep is just one sound that grabs peoples' attention when one needs it in the car.
Until it says BEEP! BEEP! BEEP!
HOW TO MAKE MY CAR BEEP
When you activate beep beep press b while driving a car, and it will honk its horn
American Cars horns beep in the tone of F
A word that suggests the source of the sound that it describes. Like beep beep for the horn of a car.
beep
By JUMPING on it
the sound that old cars make are like beep beep and (farts noises or rubles)
Tone F
An onomatopoeia is using sound words to describe something ex. "the car went beep beep! when my mom drove up"
Cars beep when you lock and unlock it so people will know if it actually worked (locked or unlocked).