The compatibility argument that never really weakened
MP3 has been around since the 1990s, and that long history means every phone, car stereo, computer, and basic media player supports it without exception — a level of universal compatibility that newer, technically superior formats like Opus or AAC still haven't fully matched, particularly on older or budget hardware. When you genuinely don't know what device or software someone will use to open a file, MP3 remains the format least likely to cause a problem.
This is why MP3 conversion requests never really went away even as better-compressing formats emerged — the question usually isn't “what's technically best” but “what will definitely work everywhere,” and MP3 still answers that question most reliably.
What quality setting actually matters
MP3's compression is controlled by bitrate, and the right choice depends entirely on content type: spoken word (podcasts, audiobooks, lectures) sounds essentially indistinguishable from higher bitrates even at 96-128kbps, while music benefits from 192-320kbps to preserve the fuller frequency range and dynamics that speech doesn't have. Using a bitrate higher than the content needs just wastes file size without any audible benefit.
320kbps is effectively MP3's practical ceiling — going higher doesn't meaningfully improve quality further, since the format's inherent limitations become the bottleneck before bitrate does.
Converting from a lossless source vs. an already-compressed one
Converting from a lossless format (WAV, FLAC) to MP3 is a single, clean compression pass — you're choosing exactly how much quality to trade for file size, starting from the best possible source. Converting from one lossy format to another (say, AAC to MP3) stacks a second round of lossy compression on top of the first, which can introduce more audible degradation than either format alone typically would.
When you have a choice, converting from the most original, highest-quality source available always produces a better result than converting from something that's already been compressed once.
When a different format is genuinely the better choice
If you're staying within Apple's ecosystem, AAC is the native format with better efficiency at equivalent quality. If you need lossless archival with no permanent quality loss, FLAC preserves everything at roughly half the size of WAV. MP3's advantage is specifically breadth of compatibility, not technical superiority — choosing it makes sense when you need that universal compatibility more than you need maximum efficiency or lossless preservation.