Traditional musical passages and scale patterns suggest a role for improvisation in folk music. The trick is to stay with what's appropriate and not range too far afield with reharmonizations or stylistic departures.

In Irish music, a "chune" can be played numerous ways. There are certain patterns such as ornamentation and rhythm that provide the unique flavor of this style of music. I think recordings of James Morrison or Micheal Coleman reveal that these masters never played the tune the same way twice.

Blues obviously requires improv. String band music Appalachian style played at hoe-downs and set-runnings could go on for hours and so improv on the tunes would relieve the boredom.

Traditional music is not fixed like classical music. It is subject to change continually.

There will be differences of opinions on how certain traditional music should be played
even with traditional musicians and singers.

Blues, Calypso, Cubano, Puerto Rican and many other forms of traditional music require
improv skills in making up lyrics as you go. Improvisation in lyrics is prized in various musical cultures.

The trick is to observe the guidelines in the style of traditional music.