This is a post about sixth chords. Someone has mentioned that sixth chords as for example F6, are not technically a part of traditional harmony. You should not read this if you are not interested in theory.
Cadences are chord progressions which establish a scale.
So three notes of the seven are established. F and A build a major third. Both choices complete the scale. These are also called cadences. For the same reason that we started with the I-V scheme.