People use FLAC over WAV or MP3 for two very important reasons.
1. FLAC a lossless codec and does not make music sound like crap which is what MP3 does. MP3 like many other countless audio codecs out there are lossy codecs.
2. FLAC is smaller than WAV files which allows you to basically store about 2x more FLAC files than WAV files on a SD card.