Always been curious…
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I assume you mean the outer part of the ear, or the “pinna?”
Short answer: to amplify and direct sound towards the inner ear, and to process incoming sound frequencies so they’ll be understandable/react to the inner ear structure.
The folds both collect and direct sound into the ear, and also “add” directional information – where the sound is coming from – through a process called “frequency dependent amplitude modulation,” the nitty-gritty of which is a little too advanced for ELI5.
The folds also amplify the sound (by as much as 10 to 15 decibels), and filter it out – with the filtering preference set to the human speech frequency range.
In short, those folds do a lot.
Think of a single microphone recording sound. It will record whatever sound reaches it, but there is no way to learn where that sound is coming from based on the recording. The same sound, made anywhere at the same distance from the microphone, will record the same (for an omni-directional microphone). For that matter, amplifying/attenuating the sound will make it appear closer/further and there’s no way to know whether loudness or distance are the cause.
We can partially solve this problem by having a second microphone separated by a known distance/orientation from the first. In this case, when a sound occurs, it will arrive at each microphone at a slightly different time and with a slightly different amplification – if I make a sound off to your left, your ‘left microphone’ will hear it first and louder than your ‘right microphone’.
Since you have two ears, this means you can do the same thing with your ears to localize sounds.
However, there’s still a problem: our two ‘microphones’ (ears) can only reduce the positioning problem to a circle perpendicular to the line between our ‘microphones’ (ears). A sound coming from directly in front of us and one coming from directly behind us will sound the same even with two ‘microphones’ (ears).
To solve *that* problem, we create a physical structure that reflects/absorbs sound waves in a known pattern that is different from every angle of arrival for the sound. Now the combination of our two ‘microphones’ (ears) and the physical reflections permit us to tell where exactly the sound is coming from in 3-dimensional space.
Note: This system isn’t perfect. It’s still possible to ‘trick’ someone’s ears (especially if you can transmit different sounds to each ear such as with headphones). But if you’re a primitive human wandering the ancient savannah, the combination of two ears and the strange shape of your ear lets you know where that lion’s roar is coming from.
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