Almost everyone who needs this conversion got here the same way: a voice note arrived from WhatsApp, Telegram or Signal, it saved as a .opus file, and nothing on the computer will open it. Opus is an excellent codec and the messaging apps are right to use it, but desktop software has been slow to catch up — default media players, older phones, most transcription tools and virtually every audio editor still expect MP3. Converting produces a file you can play, email, attach to a case file or drop into an editor without anyone needing to install a codec first.
One codec doing two very different jobs
Opus was standardised by the IETF from two separate codecs, SILK for speech and CELT for music, and it switches between them as the audio demands. That is what lets a voice note stay clear at 16 kbps, a bitrate at which MP3 is barely intelligible, and it explains the size difference you will see: a one-minute voice note is perhaps 100 KB as Opus and closer to 500 KB as a listenable MP3.
It also sets a sensible expectation for the result. Converting will not make speech recorded at a low bitrate sound better — the detail was never captured. What it gets you is a file that opens, and for a voice note that is the entire point.
The resampling nobody mentions
Opus stores everything at 48 kHz internally, regardless of what the microphone recorded at. MP3 has no 48 kHz obligation but is most commonly written at 44.1 kHz, so a conversion involves a sample-rate change on top of the codec change. The resampler handles this cleanly and it is inaudible in normal listening, but it is a real signal-processing step rather than a relabelling, which is one reason the conversion is not instant.
If you are feeding the output to transcription or forensic software that is sensitive to sample rate, it is worth checking what the tool expects. For ordinary listening, sharing and archiving, the default output is what you want.