A diatonic passing chord may be inserted into a pre-existing progression that moves by a major or minor third in order to create more movement.
[5] Passing chords may be written into a lead sheet by a composer, songwriter, or arranger.
With large ensembles, such as a big band, the comping players may have less freedom to improvise passing chords, because the composer/arranger may have already written in passing chords into the written horn parts, which might clash with improvised passing chords played by a comping musician.
The freedom of comping musicians to improvise passing chords also depends on the tempo.
In a very slow ballad, if a chord-playing musician adds in an improvised diminished chord for a half a bar, this may "clash" with the melody notes or chords played by other performers.