The major 6th chord adds a major sixth to a major triad — creating a warm, sweet, vintage sound. Before major 7th chords became popular in the 1960s, the 6th chord was the default jazz tonic voicing throughout the swing era.
The vintage jazz sound: C6 contains the same notes as Am7 (C–E–G–A = A–C–E–G). In swing, country, Hawaiian, and doo-wop, the 6th chord is the standard way to end a song with warmth rather than the dreamier maj7.