Why MP3 specifically, and not another audio format
MP3 has been the de facto standard for portable audio for over two decades, and that legacy means it plays on essentially everything — every phone, every car stereo, every basic media player, and every platform that accepts audio uploads. Extracting to MP3 rather than a newer format like AAC or Opus trades a little compression efficiency for maximum compatibility, which is exactly the right trade-off when you don't know or control what the audio will be played on.
For music, podcasts, lectures, and voice memos alike, MP3 remains the safest universal choice — the format nobody's device will ever refuse to play.
What quality to expect from the conversion
The audio quality of the resulting MP3 is capped by whatever audio was already embedded in the source MP4 — extraction can't improve on audio that was already compressed or recorded at low quality, it can only preserve what's there. At a reasonable bitrate setting (128-192kbps covers most everyday needs well), the conversion itself introduces negligible additional quality loss for typical spoken content and casual music listening.
If the source video's audio was recorded professionally at high quality, choosing a higher output bitrate preserves more of that quality; for phone-recorded video or casual content, a moderate bitrate is more than sufficient and keeps file size smaller.
The most common reasons people do this conversion
Pulling a song out of a music video, saving a podcast that was only distributed as video, converting a recorded lecture or webinar to listen to during a commute, or archiving a voice memo that was accidentally captured as video are all everyday versions of the same underlying need: the visual part isn't useful, only the sound is.
It's also common for content creators extracting a clean audio track from their own video recordings — for a separate podcast feed, for background music reuse, or simply to have a lighter-weight audio-only version of something they filmed.
One thing that surprises people about file size
A video file that felt enormous can shrink down to a genuinely tiny audio file after extraction — a 200MB video with a modest audio track might produce a 5-10MB MP3, since the video stream was responsible for the vast majority of the original size. This is often the real motivation behind the conversion even when it isn't stated explicitly: turning something too big to easily share into something trivially small.