When M4A is the right output
M4A is Apple's container for AAC-encoded audio — the format iTunes / Apple Music uses natively. AAC produces better quality than MP3 at the same bitrate (128 kbps AAC ≈ 192 kbps MP3). Convert to M4A when your destination is the Apple ecosystem (iTunes library, Apple Music sync, GarageBand, iMovie) or when file-size-per-quality matters more than universal compatibility.