FLAC files are large, and Apple Music, many car stereos and older players will not play them. AU is simple and is still read by Unix tools and Java. FLAC is at home in archiving music at full quality; AU is the usual choice for simple uncompressed audio and Unix and Java sound clips.
The defaults suit most files. Under Options you can trim the start and end.
This pair uses the compatibility engine, ffmpeg built for the browser, because AU cannot be written by the browser's built-in codecs. It is a one-off download of about 10 MB that the browser keeps, it is slower than the built-in codecs, and it takes files up to 1 GB.