A MIDI file is a score, not a recording: it stores notes, timing, velocity, instruments and tempo instructions. Phones and music apps do not all play that score the same way. This converter renders the instructions into a normal MP3 that is easy to preview, email and play anywhere.
The renderer follows tempo changes and overlapping notes, then encodes the result at 192 kbps. Its built-in instrument voices provide a consistent preview; they are not intended to duplicate the soundfont of a particular keyboard or studio workstation.
Why MIDI sounds different from MP3
MIDI describes a performance. MP3 stores the resulting sound. Converting therefore requires a synthesizer to turn every note-on, note-off, velocity and program instruction into samples. Once rendered, the MP3 no longer depends on the listener's MIDI player or instrument bank.
When to choose WAV instead
MP3 is compact and universal. If you plan to edit, mix or process the rendered track, use the MIDI to WAV converter so the first exported copy remains uncompressed. You can create the delivery MP3 after editing.