Examples of use
In major keys, Minor 7th chords works well on the II,III, and VI (2nd,3rd and 6th) degrees of the scale.
It can be used as a way of adding an extra color to a standard minor chord.
In C major, the Dm7 is the chord on II and can form part of a II⁷-V-I pattern.

In Bb major, the Dm7 is the chord on the III (3rd) degree of the scale.
This progression starts with the tonic Bb and moves to Dm7 before moving down to another minor 7th chord, Cm7, forming a progression I-III⁷-II⁷.

In F major, the Dm7 is the chord on the VI (6th) degree of the scale. Here's a common progression I-VI⁷-IV-V

Minor 7th chords are one of the Diatonic 7th chords we talk about in the theory lesson here.