It solves a few problems podcasters have,
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ambiant noice; the typical "podcast/radio" microphones are designed to reject pretty much everything besides what is in front of them, and are very good at it (within limits you may not hear the hvac system but you'll hear a door slam). The closer the microphone is the source(person), the less you have to turn up the volume. When you turn up a microphone's volume, it's going to turn up the volume of everything the microphone has captured; background noise, traffic, electrical interfernace, etc. The less you have turn up all the extra crap, the better the sound will be.
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multiple people in close promiximity. People sound different, one person might be soft spoken and the other might be loud. In order to hear the quiet person you need to crank their microphone up. If the two people shared a microphone, you'd have to find a balance between loud enough and too loud; ultimately that would lead to a viewer constantly having to adjust their volume depending on who's speaking or paying an editor to do that.
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if the microphones are farther away, the loud person's voice will get picked up by other people's mic and will cause strange sounding effects from that person getting picked up by multiple microphones.
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consider the primary purpose of a podcast; people talking for a long period of time. Sound quality has to take priority. A news interview gets away with reduced sound quality because it's for a short period of time, often less than a minute. But if they had us listening to that bad audio for an hour, most people would tune out.
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a lot of the problems I mentioned above could be fixed, or at least mitigated, with some audio editing before the episode is published. However, that doesn't help a live podcast. It also takes a long time, a 1 hour podcast with bad audio could take a day to go through it all; you have to reduce the background noice, balance everyone's audio, figure out how to make the softspoken person sound good without altering voice too while removing the background noise that's sometimes just as loud as them. It's even harder if eveyone is using 1 shared microphone, you end having to make edits everytime the speaker changes or accepting an infeiror product to save time. It's a hell of alot easier(and cheaper) to just get sound quality right from the start. With that, you might only have spend a little time to make some minor adjustments.