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 B major, the C#m7 is the chord on II and can form part of a II⁷-V-I pattern.

In A major, the C#m7 is the chord on the III (3rd) degree of the scale.
This progression starts with the tonic A and moves to C#m7 before moving down to another minor 7th chord, Bm7, forming a progression I-III⁷-II⁷.

In E major, the C#m7 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.