MP3 is "lossy"—it throws away frequencies your ear supposedly can't hear. In 1930s recordings, the frequency range is already limited. If you compress a 1935 recording to MP3, you lose the subtle details of the recording studio ambiance and the breathy lower register of Armstrong's voice. FLAC preserves every bit of data captured from the original source.
The Decca Singles 1935-1946 - Compilation by Louis Armstrong MP3 is "lossy"—it throws away frequencies your ear