FLAC files are large, and Apple Music, many car stereos and older players will not play them. WAV is uncompressed, so audio editors and transcription tools accept it without fuss. FLAC is at home in archiving music at full quality; WAV is the usual choice for recording, editing and mastering.
The defaults suit most files. Under Options you can trim the start and end.
The conversion uses your browser's own video and audio codecs (WebCodecs), with hardware acceleration where the device has it; the file is read from disk as it goes and can be up to 2 GB. Where the browser lacks a codec, the page falls back to the compatibility engine.